       1
       2
       3
       4
       5
       6
       7
       8
       9
      10
      11
      12
      13
      14
      15
      16
      17
      18
      19
      20
      21
      22
      23
      24
      25
      26
      27
      28
      29
      30
      31
      32
      33
      34
      35
      36
      37
      38
      39
      40
      41
      42
      43
      44
      45
      46
      47
      48
      49
      50
      51
      52
      53
      54
      55
      56
      57
      58
      59
      60
      61
      62
      63
      64
      65
      66
      67
      68
      69
      70
      71
      72
      73
      74
      75
      76
      77
      78
      79
      80
      81
      82
      83
      84
      85
      86
      87
      88
      89
      90
      91
      92
      93
      94
      95
      96
      97
      98
      99
     100
     101
     102
     103
     104
     105
     106
     107
     108
     109
     110
     111
     112
     113
     114
     115
     116
     117
     118
     119
     120
     121
     122
     123
     124
     125
     126
     127
     128
     129
     130
     131
     132
     133
     134
     135
     136
     137
     138
     139
     140
     141
     142
     143
     144
     145
     146
     147
     148
     149
     150
     151
     152
     153
     154
     155
     156
     157
     158
     159
     160
     161
     162
     163
     164
     165
     166
     167
     168
     169
     170
     171
     172
     173
     174
     175
     176
     177
     178
     179
     180
     181
     182
     183
     184
     185
     186
     187
     188
     189
     190
     191
     192
     193
     194
     195
     196
     197
     198
     199
     200
     201
     202
     203
     204
     205
     206
     207
     208
     209
     210
     211
     212
     213
     214
     215
     216
     217
     218
     219
     220
     221
     222
     223
     224
     225
     226
     227
     228
     229
     230
     231
     232
     233
     234
     235
     236
     237
     238
     239
     240
     241
     242
     243
     244
     245
     246
     247
     248
     249
     250
     251
     252
     253
     254
     255
     256
     257
     258
     259
     260
     261
     262
     263
     264
     265
     266
     267
     268
     269
     270
     271
     272
     273
     274
     275
     276
     277
     278
     279
     280
     281
     282
     283
     284
     285
     286
     287
     288
     289
     290
     291
     292
     293
     294
     295
     296
     297
     298
     299
     300
     301
     302
     303
     304
     305
     306
     307
     308
     309
     310
     311
     312
     313
     314
     315
     316
     317
     318
     319
     320
     321
     322
     323
     324
     325
     326
     327
     328
     329
     330
     331
     332
     333
     334
     335
     336
     337
     338
     339
     340
     341
     342
     343
     344
     345
     346
     347
     348
     349
     350
     351
     352
     353
     354
     355
     356
     357
     358
     359
     360
     361
     362
     363
     364
     365
     366
     367
     368
     369
     370
     371
     372
     373
     374
     375
     376
     377
     378
     379
     380
     381
     382
     383
     384
     385
     386
     387
     388
     389
     390
     391
     392
     393
     394
     395
     396
     397
     398
     399
     400
     401
     402
     403
     404
     405
     406
     407
     408
     409
     410
     411
     412
     413
     414
     415
     416
     417
     418
     419
     420
     421
     422
     423
     424
     425
     426
     427
     428
     429
     430
     431
     432
     433
     434
     435
     436
     437
     438
     439
     440
     441
     442
     443
     444
     445
     446
     447
     448
     449
     450
     451
     452
     453
     454
     455
     456
     457
     458
     459
     460
     461
     462
     463
     464
     465
     466
     467
     468
     469
     470
     471
     472
     473
     474
     475
     476
     477
     478
     479
     480
     481
     482
     483
     484
     485
     486
     487
     488
     489
     490
     491
     492
     493
     494
     495
     496
     497
     498
     499
     500
     501
     502
     503
     504
     505
     506
     507
     508
     509
     510
     511
     512
     513
     514
     515
     516
     517
     518
     519
     520
     521
     522
     523
     524
     525
     526
     527
     528
     529
     530
     531
     532
     533
     534
     535
     536
     537
     538
     539
     540
     541
     542
     543
     544
     545
     546
     547
     548
     549
     550
     551
     552
     553
     554
     555
     556
     557
     558
     559
     560
     561
     562
     563
     564
     565
     566
     567
     568
     569
     570
     571
     572
     573
     574
     575
     576
     577
     578
     579
     580
     581
     582
     583
     584
     585
     586
     587
     588
     589
     590
     591
     592
     593
     594
     595
     596
     597
     598
     599
     600
     601
     602
     603
     604
     605
     606
     607
     608
     609
     610
     611
     612
     613
     614
     615
     616
     617
     618
     619
     620
     621
     622
     623
     624
     625
     626
     627
     628
     629
     630
     631
     632
     633
     634
     635
     636
     637
     638
     639
     640
     641
     642
     643
     644
     645
     646
     647
     648
     649
     650
     651
     652
     653
     654
     655
     656
     657
     658
     659
     660
     661
     662
     663
     664
     665
     666
     667
     668
     669
     670
     671
     672
     673
     674
     675
     676
     677
     678
     679
     680
     681
     682
     683
     684
     685
     686
     687
     688
     689
     690
     691
     692
     693
     694
     695
     696
     697
     698
     699
     700
     701
     702
     703
     704
     705
     706
     707
     708
     709
     710
     711
     712
     713
     714
     715
     716
     717
     718
     719
     720
     721
     722
     723
     724
     725
     726
     727
     728
     729
     730
     731
     732
     733
     734
     735
     736
     737
     738
     739
     740
     741
     742
     743
     744
     745
     746
     747
     748
     749
     750
     751
     752
     753
     754
     755
     756
     757
     758
     759
     760
     761
     762
     763
     764
     765
     766
     767
     768
     769
     770
     771
     772
     773
     774
     775
     776
     777
     778
     779
     780
     781
     782
     783
     784
     785
     786
     787
     788
     789
     790
     791
     792
     793
     794
     795
     796
     797
     798
     799
     800
     801
     802
     803
     804
     805
     806
     807
     808
     809
     810
     811
     812
     813
     814
     815
     816
     817
     818
     819
     820
     821
     822
     823
     824
     825
     826
     827
     828
     829
     830
     831
     832
     833
     834
     835
     836
     837
     838
     839
     840
     841
     842
     843
     844
     845
     846
     847
     848
     849
     850
     851
     852
     853
     854
     855
     856
     857
     858
     859
     860
     861
     862
     863
     864
     865
     866
     867
     868
     869
     870
     871
     872
     873
     874
     875
     876
     877
     878
     879
     880
     881
     882
     883
     884
     885
     886
     887
     888
     889
     890
     891
     892
     893
     894
     895
     896
     897
     898
     899
     900
     901
     902
     903
     904
     905
     906
     907
     908
     909
     910
     911
     912
     913
     914
     915
     916
     917
     918
     919
     920
     921
     922
     923
     924
     925
     926
     927
     928
     929
     930
     931
     932
     933
     934
     935
     936
     937
     938
     939
     940
     941
     942
     943
     944
     945
     946
     947
     948
     949
     950
     951
     952
     953
     954
     955
     956
     957
     958
     959
     960
     961
     962
     963
     964
     965
     966
     967
     968
     969
     970
     971
     972
     973
     974
     975
     976
     977
     978
     979
     980
     981
     982
     983
     984
     985
     986
     987
     988
     989
     990
     991
     992
     993
     994
     995
     996
     997
     998
     999
    1000
    1001
    1002
    1003
    1004
    1005
    1006
    1007
    1008
    1009
    1010
    1011
    1012
    1013
    1014
    1015
    1016
    1017
    1018
    1019
    1020
    1021
    1022
    1023
    1024
    1025
    1026
    1027
    1028
    1029
    1030
    1031
    1032
    1033
    1034
    1035
    1036
    1037
    1038
    1039
    1040
    1041
    1042
    1043
    1044
    1045
    1046
    1047
    1048
    1049
    1050
    1051
    1052
    1053
    1054
    1055
    1056
    1057
    1058
    1059
    1060
    1061
    1062
    1063
    1064
    1065
    1066
    1067
    1068
    1069
    1070
    1071
    1072
    1073
    1074
    1075
    1076
    1077
    1078
    1079
    1080
    1081
    1082
    1083
    1084
    1085
    1086
    1087
    1088
    1089
    1090
    1091
    1092
    1093
    1094
    1095
    1096
    1097
    1098
    1099
    1100
    1101
    1102
    1103
    1104
    1105
    1106
    1107
    1108
    1109
    1110
    1111
    1112
    1113
    1114
    1115
    1116
    1117
    1118
    1119
    1120
    1121
    1122
    1123
    1124
    1125
    1126
    1127
    1128
    1129
    1130
    1131
    1132
    1133
    1134
    1135
    1136
    1137
    1138
    1139
    1140
    1141
    1142
    1143
    1144
    1145
    1146
    1147
    1148
    1149
    1150
    1151
    1152
    1153
    1154
    1155
    1156
    1157
    1158
    1159
    1160
    1161
    1162
    1163
    1164
    1165
    1166
    1167
    1168
    1169
    1170
    1171
    1172

    // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Error handling with the `Result` type.
    //!
    //! [`Result<T, E>`][`Result`] is the type used for returning and propagating
    //! errors. It is an enum with the variants, [`Ok(T)`], representing
    //! success and containing a value, and [`Err(E)`], representing error
    //! and containing an error value.
    //!
    //! ```
    //! # #[allow(dead_code)]
    //! enum Result<T, E> {
    //!    Ok(T),
    //!    Err(E),
    //! }
    //! ```
    //!
    //! Functions return [`Result`] whenever errors are expected and
    //! recoverable. In the `std` crate, [`Result`] is most prominently used
    //! for [I/O](../../std/io/index.html).
    //!
    //! A simple function returning [`Result`] might be
    //! defined and used like so:
    //!
    //! ```
    //! #[derive(Debug)]
    //! enum Version { Version1, Version2 }
    //!
    //! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
    //!     match header.get(0) {
    //!         None => Err("invalid header length"),
    //!         Some(&1) => Ok(Version::Version1),
    //!         Some(&2) => Ok(Version::Version2),
    //!         Some(_) => Err("invalid version"),
    //!     }
    //! }
    //!
    //! let version = parse_version(&[1, 2, 3, 4]);
    //! match version {
    //!     Ok(v) => println!("working with version: {:?}", v),
    //!     Err(e) => println!("error parsing header: {:?}", e),
    //! }
    //! ```
    //!
    //! Pattern matching on [`Result`]s is clear and straightforward for
    //! simple cases, but [`Result`] comes with some convenience methods
    //! that make working with it more succinct.
    //!
    //! ```
    //! let good_result: Result<i32, i32> = Ok(10);
    //! let bad_result: Result<i32, i32> = Err(10);
    //!
    //! // The `is_ok` and `is_err` methods do what they say.
    //! assert!(good_result.is_ok() && !good_result.is_err());
    //! assert!(bad_result.is_err() && !bad_result.is_ok());
    //!
    //! // `map` consumes the `Result` and produces another.
    //! let good_result: Result<i32, i32> = good_result.map(|i| i + 1);
    //! let bad_result: Result<i32, i32> = bad_result.map(|i| i - 1);
    //!
    //! // Use `and_then` to continue the computation.
    //! let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));
    //!
    //! // Use `or_else` to handle the error.
    //! let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));
    //!
    //! // Consume the result and return the contents with `unwrap`.
    //! let final_awesome_result = good_result.unwrap();
    //! ```
    //!
    //! # Results must be used
    //!
    //! A common problem with using return values to indicate errors is
    //! that it is easy to ignore the return value, thus failing to handle
    //! the error. [`Result`] is annotated with the `#[must_use]` attribute,
    //! which will cause the compiler to issue a warning when a Result
    //! value is ignored. This makes [`Result`] especially useful with
    //! functions that may encounter errors but don't otherwise return a
    //! useful value.
    //!
    //! Consider the [`write_all`] method defined for I/O types
    //! by the [`Write`] trait:
    //!
    //! ```
    //! use std::io;
    //!
    //! trait Write {
    //!     fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
    //! }
    //! ```
    //!
    //! *Note: The actual definition of [`Write`] uses [`io::Result`], which
    //! is just a synonym for [`Result`]`<T, `[`io::Error`]`>`.*
    //!
    //! This method doesn't produce a value, but the write may
    //! fail. It's crucial to handle the error case, and *not* write
    //! something like this:
    //!
    //! ```no_run
    //! # #![allow(unused_must_use)] // \o/
    //! use std::fs::File;
    //! use std::io::prelude::*;
    //!
    //! let mut file = File::create("valuable_data.txt").unwrap();
    //! // If `write_all` errors, then we'll never know, because the return
    //! // value is ignored.
    //! file.write_all(b"important message");
    //! ```
    //!
    //! If you *do* write that in Rust, the compiler will give you a
    //! warning (by default, controlled by the `unused_must_use` lint).
    //!
    //! You might instead, if you don't want to handle the error, simply
    //! assert success with [`expect`]. This will panic if the
    //! write fails, providing a marginally useful message indicating why:
    //!
    //! ```{.no_run}
    //! use std::fs::File;
    //! use std::io::prelude::*;
    //!
    //! let mut file = File::create("valuable_data.txt").unwrap();
    //! file.write_all(b"important message").expect("failed to write message");
    //! ```
    //!
    //! You might also simply assert success:
    //!
    //! ```{.no_run}
    //! # use std::fs::File;
    //! # use std::io::prelude::*;
    //! # let mut file = File::create("valuable_data.txt").unwrap();
    //! assert!(file.write_all(b"important message").is_ok());
    //! ```
    //!
    //! Or propagate the error up the call stack with [`?`]:
    //!
    //! ```
    //! # use std::fs::File;
    //! # use std::io::prelude::*;
    //! # use std::io;
    //! # #[allow(dead_code)]
    //! fn write_message() -> io::Result<()> {
    //!     let mut file = File::create("valuable_data.txt")?;
    //!     file.write_all(b"important message")?;
    //!     Ok(())
    //! }
    //! ```
    //!
    //! # The `?` syntax
    //!
    //! When writing code that calls many functions that return the
    //! [`Result`] type, the error handling can be tedious. The [`?`]
    //! syntax hides some of the boilerplate of propagating errors up the
    //! call stack.
    //!
    //! It replaces this:
    //!
    //! ```
    //! # #![allow(dead_code)]
    //! use std::fs::File;
    //! use std::io::prelude::*;
    //! use std::io;
    //!
    //! struct Info {
    //!     name: String,
    //!     age: i32,
    //!     rating: i32,
    //! }
    //!
    //! fn write_info(info: &Info) -> io::Result<()> {
    //!     // Early return on error
    //!     let mut file = match File::create("my_best_friends.txt") {
    //!            Err(e) => return Err(e),
    //!            Ok(f) => f,
    //!     };
    //!     if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
    //!         return Err(e)
    //!     }
    //!     if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
    //!         return Err(e)
    //!     }
    //!     if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
    //!         return Err(e)
    //!     }
    //!     Ok(())
    //! }
    //! ```
    //!
    //! With this:
    //!
    //! ```
    //! # #![allow(dead_code)]
    //! use std::fs::File;
    //! use std::io::prelude::*;
    //! use std::io;
    //!
    //! struct Info {
    //!     name: String,
    //!     age: i32,
    //!     rating: i32,
    //! }
    //!
    //! fn write_info(info: &Info) -> io::Result<()> {
    //!     let mut file = File::create("my_best_friends.txt")?;
    //!     // Early return on error
    //!     file.write_all(format!("name: {}\n", info.name).as_bytes())?;
    //!     file.write_all(format!("age: {}\n", info.age).as_bytes())?;
    //!     file.write_all(format!("rating: {}\n", info.rating).as_bytes())?;
    //!     Ok(())
    //! }
    //! ```
    //!
    //! *It's much nicer!*
    //!
    //! Ending the expression with [`?`] will result in the unwrapped
    //! success ([`Ok`]) value, unless the result is [`Err`], in which case
    //! [`Err`] is returned early from the enclosing function.
    //!
    //! [`?`] can only be used in functions that return [`Result`] because of the
    //! early return of [`Err`] that it provides.
    //!
    //! [`expect`]: enum.Result.html#method.expect
    //! [`Write`]: ../../std/io/trait.Write.html
    //! [`write_all`]: ../../std/io/trait.Write.html#method.write_all
    //! [`io::Result`]: ../../std/io/type.Result.html
    //! [`?`]: ../../std/macro.try.html
    //! [`Result`]: enum.Result.html
    //! [`Ok(T)`]: enum.Result.html#variant.Ok
    //! [`Err(E)`]: enum.Result.html#variant.Err
    //! [`io::Error`]: ../../std/io/struct.Error.html
    //! [`Ok`]: enum.Result.html#variant.Ok
    //! [`Err`]: enum.Result.html#variant.Err

    #![stable(feature = "rust1", since = "1.0.0")]

    use fmt;
    use iter::{FromIterator, FusedIterator, TrustedLen};
    use ops;

    /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
    ///
    /// See the [`std::result`](index.html) module documentation for details.
    ///
    /// [`Ok`]: enum.Result.html#variant.Ok
    /// [`Err`]: enum.Result.html#variant.Err
    #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub enum Result<T, E> {
        /// Contains the success value
        #[stable(feature = "rust1", since = "1.0.0")]
        Ok(#[stable(feature = "rust1", since = "1.0.0")] T),

        /// Contains the error value
        #[stable(feature = "rust1", since = "1.0.0")]
        Err(#[stable(feature = "rust1", since = "1.0.0")] E),
    }

    /////////////////////////////////////////////////////////////////////////////
    // Type implementation
    /////////////////////////////////////////////////////////////////////////////

    impl<T, E> Result<T, E> {
        /////////////////////////////////////////////////////////////////////////
        // Querying the contained values
        /////////////////////////////////////////////////////////////////////////

        /// Returns `true` if the result is [`Ok`].
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let x: Result<i32, &str> = Ok(-3);
        /// assert_eq!(x.is_ok(), true);
        ///
        /// let x: Result<i32, &str> = Err("Some error message");
        /// assert_eq!(x.is_ok(), false);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_ok(&self) -> bool {
            match *self {
                Ok(_) => true,
                Err(_) => false
            }
        }

        /// Returns `true` if the result is [`Err`].
        ///
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let x: Result<i32, &str> = Ok(-3);
        /// assert_eq!(x.is_err(), false);
        ///
        /// let x: Result<i32, &str> = Err("Some error message");
        /// assert_eq!(x.is_err(), true);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_err(&self) -> bool {
            !self.is_ok()
        }

        /////////////////////////////////////////////////////////////////////////
        // Adapter for each variant
        /////////////////////////////////////////////////////////////////////////

        /// Converts from `Result<T, E>` to [`Option<T>`].
        ///
        /// Converts `self` into an [`Option<T>`], consuming `self`,
        /// and discarding the error, if any.
        ///
        /// [`Option<T>`]: ../../std/option/enum.Option.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(2);
        /// assert_eq!(x.ok(), Some(2));
        ///
        /// let x: Result<u32, &str> = Err("Nothing here");
        /// assert_eq!(x.ok(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn ok(self) -> Option<T> {
            match self {
                Ok(x)  => Some(x),
                Err(_) => None,
            }
        }

        /// Converts from `Result<T, E>` to [`Option<E>`].
        ///
        /// Converts `self` into an [`Option<E>`], consuming `self`,
        /// and discarding the success value, if any.
        ///
        /// [`Option<E>`]: ../../std/option/enum.Option.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(2);
        /// assert_eq!(x.err(), None);
        ///
        /// let x: Result<u32, &str> = Err("Nothing here");
        /// assert_eq!(x.err(), Some("Nothing here"));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn err(self) -> Option<E> {
            match self {
                Ok(_)  => None,
                Err(x) => Some(x),
            }
        }

        /////////////////////////////////////////////////////////////////////////
        // Adapter for working with references
        /////////////////////////////////////////////////////////////////////////

        /// Converts from `Result<T, E>` to `Result<&T, &E>`.
        ///
        /// Produces a new `Result`, containing a reference
        /// into the original, leaving the original in place.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(2);
        /// assert_eq!(x.as_ref(), Ok(&2));
        ///
        /// let x: Result<u32, &str> = Err("Error");
        /// assert_eq!(x.as_ref(), Err(&"Error"));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn as_ref(&self) -> Result<&T, &E> {
            match *self {
                Ok(ref x) => Ok(x),
                Err(ref x) => Err(x),
            }
        }

        /// Converts from `Result<T, E>` to `Result<&mut T, &mut E>`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// fn mutate(r: &mut Result<i32, i32>) {
        ///     match r.as_mut() {
        ///         Ok(v) => *v = 42,
        ///         Err(e) => *e = 0,
        ///     }
        /// }
        ///
        /// let mut x: Result<i32, i32> = Ok(2);
        /// mutate(&mut x);
        /// assert_eq!(x.unwrap(), 42);
        ///
        /// let mut x: Result<i32, i32> = Err(13);
        /// mutate(&mut x);
        /// assert_eq!(x.unwrap_err(), 0);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn as_mut(&mut self) -> Result<&mut T, &mut E> {
            match *self {
                Ok(ref mut x) => Ok(x),
                Err(ref mut x) => Err(x),
            }
        }

        /////////////////////////////////////////////////////////////////////////
        // Transforming contained values
        /////////////////////////////////////////////////////////////////////////

        /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
        /// contained [`Ok`] value, leaving an [`Err`] value untouched.
        ///
        /// This function can be used to compose the results of two functions.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Print the numbers on each line of a string multiplied by two.
        ///
        /// ```
        /// let line = "1\n2\n3\n4\n";
        ///
        /// for num in line.lines() {
        ///     match num.parse::<i32>().map(|i| i * 2) {
        ///         Ok(n) => println!("{}", n),
        ///         Err(..) => {}
        ///     }
        /// }
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U,E> {
            match self {
                Ok(t) => Ok(op(t)),
                Err(e) => Err(e)
            }
        }

        /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
        /// contained [`Err`] value, leaving an [`Ok`] value untouched.
        ///
        /// This function can be used to pass through a successful result while handling
        /// an error.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
        ///
        /// let x: Result<u32, u32> = Ok(2);
        /// assert_eq!(x.map_err(stringify), Ok(2));
        ///
        /// let x: Result<u32, u32> = Err(13);
        /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T,F> {
            match self {
                Ok(t) => Ok(t),
                Err(e) => Err(op(e))
            }
        }

        /////////////////////////////////////////////////////////////////////////
        // Iterator constructors
        /////////////////////////////////////////////////////////////////////////

        /// Returns an iterator over the possibly contained value.
        ///
        /// The iterator yields one value if the result is [`Ok`], otherwise none.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(7);
        /// assert_eq!(x.iter().next(), Some(&7));
        ///
        /// let x: Result<u32, &str> = Err("nothing!");
        /// assert_eq!(x.iter().next(), None);
        /// ```
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter(&self) -> Iter<T> {
            Iter { inner: self.as_ref().ok() }
        }

        /// Returns a mutable iterator over the possibly contained value.
        ///
        /// The iterator yields one value if the result is [`Ok`], otherwise none.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut x: Result<u32, &str> = Ok(7);
        /// match x.iter_mut().next() {
        ///     Some(v) => *v = 40,
        ///     None => {},
        /// }
        /// assert_eq!(x, Ok(40));
        ///
        /// let mut x: Result<u32, &str> = Err("nothing!");
        /// assert_eq!(x.iter_mut().next(), None);
        /// ```
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter_mut(&mut self) -> IterMut<T> {
            IterMut { inner: self.as_mut().ok() }
        }

        ////////////////////////////////////////////////////////////////////////
        // Boolean operations on the values, eager and lazy
        /////////////////////////////////////////////////////////////////////////

        /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(2);
        /// let y: Result<&str, &str> = Err("late error");
        /// assert_eq!(x.and(y), Err("late error"));
        ///
        /// let x: Result<u32, &str> = Err("early error");
        /// let y: Result<&str, &str> = Ok("foo");
        /// assert_eq!(x.and(y), Err("early error"));
        ///
        /// let x: Result<u32, &str> = Err("not a 2");
        /// let y: Result<&str, &str> = Err("late error");
        /// assert_eq!(x.and(y), Err("not a 2"));
        ///
        /// let x: Result<u32, &str> = Ok(2);
        /// let y: Result<&str, &str> = Ok("different result type");
        /// assert_eq!(x.and(y), Ok("different result type"));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
            match self {
                Ok(_) => res,
                Err(e) => Err(e),
            }
        }

        /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// This function can be used for control flow based on `Result` values.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
        /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
        ///
        /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
        /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
        /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
        /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
            match self {
                Ok(t) => op(t),
                Err(e) => Err(e),
            }
        }

        /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(2);
        /// let y: Result<u32, &str> = Err("late error");
        /// assert_eq!(x.or(y), Ok(2));
        ///
        /// let x: Result<u32, &str> = Err("early error");
        /// let y: Result<u32, &str> = Ok(2);
        /// assert_eq!(x.or(y), Ok(2));
        ///
        /// let x: Result<u32, &str> = Err("not a 2");
        /// let y: Result<u32, &str> = Err("late error");
        /// assert_eq!(x.or(y), Err("late error"));
        ///
        /// let x: Result<u32, &str> = Ok(2);
        /// let y: Result<u32, &str> = Ok(100);
        /// assert_eq!(x.or(y), Ok(2));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
            match self {
                Ok(v) => Ok(v),
                Err(_) => res,
            }
        }

        /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
        ///
        /// This function can be used for control flow based on result values.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
        /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
        ///
        /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
        /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
        /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
        /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
            match self {
                Ok(t) => Ok(t),
                Err(e) => op(e),
            }
        }

        /// Unwraps a result, yielding the content of an [`Ok`].
        /// Else, it returns `optb`.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let optb = 2;
        /// let x: Result<u32, &str> = Ok(9);
        /// assert_eq!(x.unwrap_or(optb), 9);
        ///
        /// let x: Result<u32, &str> = Err("error");
        /// assert_eq!(x.unwrap_or(optb), optb);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unwrap_or(self, optb: T) -> T {
            match self {
                Ok(t) => t,
                Err(_) => optb
            }
        }

        /// Unwraps a result, yielding the content of an [`Ok`].
        /// If the value is an [`Err`] then it calls `op` with its value.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// fn count(x: &str) -> usize { x.len() }
        ///
        /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
        /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
            match self {
                Ok(t) => t,
                Err(e) => op(e)
            }
        }
    }

    impl<T, E: fmt::Debug> Result<T, E> {
        /// Unwraps a result, yielding the content of an [`Ok`].
        ///
        /// # Panics
        ///
        /// Panics if the value is an [`Err`], with a panic message provided by the
        /// [`Err`]'s value.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(2);
        /// assert_eq!(x.unwrap(), 2);
        /// ```
        ///
        /// ```{.should_panic}
        /// let x: Result<u32, &str> = Err("emergency failure");
        /// x.unwrap(); // panics with `emergency failure`
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unwrap(self) -> T {
            match self {
                Ok(t) => t,
                Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", e),
            }
        }

        /// Unwraps a result, yielding the content of an [`Ok`].
        ///
        /// # Panics
        ///
        /// Panics if the value is an [`Err`], with a panic message including the
        /// passed message, and the content of the [`Err`].
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```{.should_panic}
        /// let x: Result<u32, &str> = Err("emergency failure");
        /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
        /// ```
        #[inline]
        #[stable(feature = "result_expect", since = "1.4.0")]
        pub fn expect(self, msg: &str) -> T {
            match self {
                Ok(t) => t,
                Err(e) => unwrap_failed(msg, e),
            }
        }
    }

    impl<T: fmt::Debug, E> Result<T, E> {
        /// Unwraps a result, yielding the content of an [`Err`].
        ///
        /// # Panics
        ///
        /// Panics if the value is an [`Ok`], with a custom panic message provided
        /// by the [`Ok`]'s value.
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        ///
        /// # Examples
        ///
        /// ```{.should_panic}
        /// let x: Result<u32, &str> = Ok(2);
        /// x.unwrap_err(); // panics with `2`
        /// ```
        ///
        /// ```
        /// let x: Result<u32, &str> = Err("emergency failure");
        /// assert_eq!(x.unwrap_err(), "emergency failure");
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unwrap_err(self) -> E {
            match self {
                Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", t),
                Err(e) => e,
            }
        }

        /// Unwraps a result, yielding the content of an [`Err`].
        ///
        /// # Panics
        ///
        /// Panics if the value is an [`Ok`], with a panic message including the
        /// passed message, and the content of the [`Ok`].
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```{.should_panic}
        /// let x: Result<u32, &str> = Ok(10);
        /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
        /// ```
        #[inline]
        #[stable(feature = "result_expect_err", since = "1.17.0")]
        pub fn expect_err(self, msg: &str) -> E {
            match self {
                Ok(t) => unwrap_failed(msg, t),
                Err(e) => e,
            }
        }
    }

    impl<T: Default, E> Result<T, E> {
        /// Returns the contained value or a default
        ///
        /// Consumes the `self` argument then, if [`Ok`], returns the contained
        /// value, otherwise if [`Err`], returns the default value for that
        /// type.
        ///
        /// # Examples
        ///
        /// Convert a string to an integer, turning poorly-formed strings
        /// into 0 (the default value for integers). [`parse`] converts
        /// a string to any other type that implements [`FromStr`], returning an
        /// [`Err`] on error.
        ///
        /// ```
        /// let good_year_from_input = "1909";
        /// let bad_year_from_input = "190blarg";
        /// let good_year = good_year_from_input.parse().unwrap_or_default();
        /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
        ///
        /// assert_eq!(1909, good_year);
        /// assert_eq!(0, bad_year);
        /// ```
        ///
        /// [`parse`]: ../../std/primitive.str.html#method.parse
        /// [`FromStr`]: ../../std/str/trait.FromStr.html
        /// [`Ok`]: enum.Result.html#variant.Ok
        /// [`Err`]: enum.Result.html#variant.Err
        #[inline]
        #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
        pub fn unwrap_or_default(self) -> T {
            match self {
                Ok(x) => x,
                Err(_) => Default::default(),
            }
        }
    }

    // This is a separate function to reduce the code size of the methods
    #[inline(never)]
    #[cold]
    fn unwrap_failed<E: fmt::Debug>(msg: &str, error: E) -> ! {
        panic!("{}: {:?}", msg, error)
    }

    /////////////////////////////////////////////////////////////////////////////
    // Trait implementations
    /////////////////////////////////////////////////////////////////////////////

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T, E> IntoIterator for Result<T, E> {
        type Item = T;
        type IntoIter = IntoIter<T>;

        /// Returns a consuming iterator over the possibly contained value.
        ///
        /// The iterator yields one value if the result is [`Ok`], otherwise none.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let x: Result<u32, &str> = Ok(5);
        /// let v: Vec<u32> = x.into_iter().collect();
        /// assert_eq!(v, [5]);
        ///
        /// let x: Result<u32, &str> = Err("nothing!");
        /// let v: Vec<u32> = x.into_iter().collect();
        /// assert_eq!(v, []);
        /// ```
        ///
        /// [`Ok`]: enum.Result.html#variant.Ok
        #[inline]
        fn into_iter(self) -> IntoIter<T> {
            IntoIter { inner: self.ok() }
        }
    }

    #[stable(since = "1.4.0", feature = "result_iter")]
    impl<'a, T, E> IntoIterator for &'a Result<T, E> {
        type Item = &'a T;
        type IntoIter = Iter<'a, T>;

        fn into_iter(self) -> Iter<'a, T> {
            self.iter()
        }
    }

    #[stable(since = "1.4.0", feature = "result_iter")]
    impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
        type Item = &'a mut T;
        type IntoIter = IterMut<'a, T>;

        fn into_iter(self) -> IterMut<'a, T> {
            self.iter_mut()
        }
    }

    /////////////////////////////////////////////////////////////////////////////
    // The Result Iterators
    /////////////////////////////////////////////////////////////////////////////

    /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
    ///
    /// The iterator yields one value if the result is [`Ok`], otherwise none.
    ///
    /// Created by [`Result::iter`].
    ///
    /// [`Ok`]: enum.Result.html#variant.Ok
    /// [`Result`]: enum.Result.html
    /// [`Result::iter`]: enum.Result.html#method.iter
    #[derive(Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Iter<'a, T: 'a> { inner: Option<&'a T> }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Iterator for Iter<'a, T> {
        type Item = &'a T;

        #[inline]
        fn next(&mut self) -> Option<&'a T> { self.inner.take() }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let n = if self.inner.is_some() {1} else {0};
            (n, Some(n))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
        #[inline]
        fn next_back(&mut self) -> Option<&'a T> { self.inner.take() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> ExactSizeIterator for Iter<'a, T> {}

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, T> FusedIterator for Iter<'a, T> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<'a, A> TrustedLen for Iter<'a, A> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Clone for Iter<'a, T> {
        fn clone(&self) -> Iter<'a, T> { Iter { inner: self.inner } }
    }

    /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
    ///
    /// Created by [`Result::iter_mut`].
    ///
    /// [`Ok`]: enum.Result.html#variant.Ok
    /// [`Result`]: enum.Result.html
    /// [`Result::iter_mut`]: enum.Result.html#method.iter_mut
    #[derive(Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IterMut<'a, T: 'a> { inner: Option<&'a mut T> }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Iterator for IterMut<'a, T> {
        type Item = &'a mut T;

        #[inline]
        fn next(&mut self) -> Option<&'a mut T> { self.inner.take() }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let n = if self.inner.is_some() {1} else {0};
            (n, Some(n))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
        #[inline]
        fn next_back(&mut self) -> Option<&'a mut T> { self.inner.take() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> ExactSizeIterator for IterMut<'a, T> {}

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, T> FusedIterator for IterMut<'a, T> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<'a, A> TrustedLen for IterMut<'a, A> {}

    /// An iterator over the value in a [`Ok`] variant of a [`Result`].
    ///
    /// The iterator yields one value if the result is [`Ok`], otherwise none.
    ///
    /// This struct is created by the [`into_iter`] method on
    /// [`Result`][`Result`] (provided by the [`IntoIterator`] trait).
    ///
    /// [`Ok`]: enum.Result.html#variant.Ok
    /// [`Result`]: enum.Result.html
    /// [`into_iter`]: ../iter/trait.IntoIterator.html#tymethod.into_iter
    /// [`IntoIterator`]: ../iter/trait.IntoIterator.html
    #[derive(Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IntoIter<T> { inner: Option<T> }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Iterator for IntoIter<T> {
        type Item = T;

        #[inline]
        fn next(&mut self) -> Option<T> { self.inner.take() }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let n = if self.inner.is_some() {1} else {0};
            (n, Some(n))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> DoubleEndedIterator for IntoIter<T> {
        #[inline]
        fn next_back(&mut self) -> Option<T> { self.inner.take() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ExactSizeIterator for IntoIter<T> {}

    #[unstable(feature = "fused", issue = "35602")]
    impl<T> FusedIterator for IntoIter<T> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A> TrustedLen for IntoIter<A> {}

    /////////////////////////////////////////////////////////////////////////////
    // FromIterator
    /////////////////////////////////////////////////////////////////////////////

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
        /// Takes each element in the `Iterator`: if it is an `Err`, no further
        /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
        /// container with the values of each `Result` is returned.
        ///
        /// Here is an example which increments every integer in a vector,
        /// checking for overflow:
        ///
        /// ```
        /// let v = vec![1, 2];
        /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
        ///     x.checked_add(1).ok_or("Overflow!")
        /// ).collect();
        /// assert!(res == Ok(vec![2, 3]));
        /// ```
        #[inline]
        fn from_iter<I: IntoIterator<Item=Result<A, E>>>(iter: I) -> Result<V, E> {
            // FIXME(#11084): This could be replaced with Iterator::scan when this
            // performance bug is closed.

            struct Adapter<Iter, E> {
                iter: Iter,
                err: Option<E>,
            }

            impl<T, E, Iter: Iterator<Item=Result<T, E>>> Iterator for Adapter<Iter, E> {
                type Item = T;

                #[inline]
                fn next(&mut self) -> Option<T> {
                    match self.iter.next() {
                        Some(Ok(value)) => Some(value),
                        Some(Err(err)) => {
                            self.err = Some(err);
                            None
                        }
                        None => None,
                    }
                }

                fn size_hint(&self) -> (usize, Option<usize>) {
                    let (_min, max) = self.iter.size_hint();
                    (0, max)
                }
            }

            let mut adapter = Adapter { iter: iter.into_iter(), err: None };
            let v: V = FromIterator::from_iter(adapter.by_ref());

            match adapter.err {
                Some(err) => Err(err),
                None => Ok(v),
            }
        }
    }

    #[unstable(feature = "try_trait", issue = "42327")]
    impl<T,E> ops::Try for Result<T, E> {
        type Ok = T;
        type Error = E;

        fn into_result(self) -> Self {
            self
        }

        fn from_ok(v: T) -> Self {
            Ok(v)
        }

        fn from_error(v: E) -> Self {
            Err(v)
        }
    }

# Help

## Keyboard Shortcuts

?
  Show this help dialog
S
  Focus the search field
⇤
  Move up in search results
⇥
  Move down in search results
⏎
  Go to active search result
+
  Collapse/expand all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g. `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g. `vec -> usize` or `* -> vec`)