lens-4.13: Lenses, Folds and Traversals

Copyright | (C) 2012-15 Edward Kmett, 2012-13 Michael Sloan
License | BSD-style (see the file LICENSE)
Maintainer | Edward Kmett <ekmett@gmail.com>
Stability | experimental
Portability | non-portable
Safe Haskell | Trustworthy
Language | Haskell98

Control.Lens.TH

Contents

- Constructing Lenses Automatically
- Constructing Lenses Given a Declaration Quote
- Configuring Lenses

Description

Synopsis

- makeLenses :: Name -> DecsQ
- makeLensesFor :: [(String, String)] -> Name -> DecsQ
- makeClassy :: Name -> DecsQ
- makeClassyFor :: String -> String -> [(String, String)] -> Name -> DecsQ
- makeClassy_ :: Name -> DecsQ
- makePrisms :: Name -> DecsQ
- makeClassyPrisms :: Name -> DecsQ
- makeWrapped :: Name -> DecsQ
- makeFields :: Name -> DecsQ
- declareLenses :: DecsQ -> DecsQ
- declareLensesFor :: [(String, String)] -> DecsQ -> DecsQ
- declareClassy :: DecsQ -> DecsQ
- declareClassyFor :: [(String, (String, String))] -> [(String, String)] -> DecsQ -> DecsQ
- declarePrisms :: DecsQ -> DecsQ
- declareWrapped :: DecsQ -> DecsQ
- declareFields :: DecsQ -> DecsQ
- makeLensesWith :: LensRules -> Name -> DecsQ
- declareLensesWith :: LensRules -> DecsQ -> DecsQ
- defaultFieldRules :: LensRules
- camelCaseFields :: LensRules
- underscoreFields :: LensRules
- abbreviatedFields :: LensRules
- data LensRules
- data DefName
  - = TopName Name
  - | MethodName Name Name
- lensRules :: LensRules
- lensRulesFor :: [(String, String)] -> LensRules
- classyRules :: LensRules
- classyRules_ :: LensRules
- lensField :: Lens' LensRules (Name -> [Name] -> Name -> [DefName])
- lensClass :: Lens' LensRules (Name -> Maybe (Name, Name))
- simpleLenses :: Lens' LensRules Bool
- createClass :: Lens' LensRules Bool
- generateSignatures :: Lens' LensRules Bool
- generateUpdateableOptics :: Lens' LensRules Bool
- generateLazyPatterns :: Lens' LensRules Bool

# Constructing Lenses Automatically

makeLenses :: Name -> DecsQ Source

Build lenses (and traversals) with a sensible default configuration.

e.g.

    data FooBar
      = Foo { _x, _y :: Int }
      | Bar { _x :: Int }
    makeLenses ''FooBar

will create

    x :: Lens' FooBar Int
    x f (Foo a b) = (\a' -> Foo a' b) <$> f a
    x f (Bar a)   = Bar <$> f a
    y :: Traversal' FooBar Int
    y f (Foo a b) = (\b' -> Foo a  b') <$> f b
    y _ c@(Bar _) = pure c

    makeLenses = makeLensesWith lensRules

makeLensesFor :: [(String, String)] -> Name -> DecsQ Source

Derive lenses and traversals, specifying explicit pairings of `(fieldName, lensName)`.

If you map multiple names to the same label, and it is present in the same constructor then this will generate a `Traversal`.

e.g.

    makeLensesFor [("_foo", "fooLens"), ("baz", "lbaz")] ''Foo
    makeLensesFor [("_barX", "bar"), ("_barY", "bar")] ''Bar

makeClassy :: Name -> DecsQ Source

Make lenses and traversals for a type, and create a class when the type has no arguments.

e.g.

    data Foo = Foo { _fooX, _fooY :: Int }
    makeClassy ''Foo

will create

    class HasFoo t where
      foo :: Lens' t Foo
      fooX :: Lens' t Int
      fooX = foo . go where go f (Foo x y) = (\x' -> Foo x' y) <$> f x
      fooY :: Lens' t Int
      fooY = foo . go where go f (Foo x y) = (\y' -> Foo x y') <$> f y
    instance HasFoo Foo where
      foo = id

    makeClassy = makeLensesWith classyRules

makeClassyFor :: String -> String -> [(String, String)] -> Name -> DecsQ Source

Derive lenses and traversals, using a named wrapper class, and specifying explicit pairings of `(fieldName, traversalName)`.

Example usage:

    makeClassyFor "HasFoo" "foo" [("_foo", "fooLens"), ("bar", "lbar")] ''Foo

makeClassy_ :: Name -> DecsQ Source

Make lenses and traversals for a type, and create a class when the type has no arguments. Works the same as `makeClassy` except that (a) it expects that record field names do not begin with an underscore, (b) all record fields are made into lenses, and (c) the resulting lens is prefixed with an underscore.

makePrisms Source

Arguments

:: Name | Type constructor name
-> DecsQ |

Generate a `Prism` for each constructor of a data type. Isos generated when possible. Reviews are created for constructors with existentially quantified constructors and GADTs.

e.g.

    data FooBarBaz a
      = Foo Int
      | Bar a
      | Baz Int Char
    makePrisms ''FooBarBaz

will create

    _Foo :: Prism' (FooBarBaz a) Int
    _Bar :: Prism (FooBarBaz a) (FooBarBaz b) a b
    _Baz :: Prism' (FooBarBaz a) (Int, Char)

makeClassyPrisms Source

Arguments

:: Name | Type constructor name
-> DecsQ |

Generate a `Prism` for each constructor of a data type and combine them into a single class. No Isos are created. Reviews are created for constructors with existentially quantified constructors and GADTs.

e.g.

    data FooBarBaz a
      = Foo Int
      | Bar a
      | Baz Int Char
    makeClassyPrisms ''FooBarBaz

will create

    class AsFooBarBaz s a | s -> a where
      _FooBarBaz :: Prism' s (FooBarBaz a)
      _Foo :: Prism' s Int
      _Bar :: Prism' s a
      _Baz :: Prism' s (Int,Char)

      _Foo = _FooBarBaz . _Foo
      _Bar = _FooBarBaz . _Bar
      _Baz = _FooBarBaz . _Baz

    instance AsFooBarBaz (FooBarBaz a) a

Generate an As class of prisms. Names are selected by prefixing the constructor name with an underscore. Constructors with multiple fields will construct Prisms to tuples of those fields.

makeWrapped :: Name -> DecsQ Source

Build `Wrapped` instance for a given newtype

makeFields :: Name -> DecsQ Source

Generate overloaded field accessors.

e.g

    data Foo a = Foo { _fooX :: Int, _fooY : a }
    newtype Bar = Bar { _barX :: Char }
    makeFields ''Foo
    makeFields ''Bar

will create

    _fooXLens :: Lens' (Foo a) Int
    _fooYLens :: Lens (Foo a) (Foo b) a b
    class HasX s a | s -> a where
      x :: Lens' s a
    instance HasX (Foo a) Int where
      x = _fooXLens
    class HasY s a | s -> a where
      y :: Lens' s a
    instance HasY (Foo a) a where
      y = _fooYLens
    _barXLens :: Iso' Bar Char
    instance HasX Bar Char where
      x = _barXLens

For details, see `camelCaseFields`.

    makeFields = makeLensesWith defaultFieldRules

# Constructing Lenses Given a Declaration Quote

declareLenses :: DecsQ -> DecsQ Source

Make lenses for all records in the given declaration quote. All record syntax in the input will be stripped off.

e.g.

    declareLenses [d|
      data Foo = Foo { fooX, fooY :: Int }
        deriving Show
      |]

will create

    data Foo = Foo Int Int deriving Show
    fooX, fooY :: Lens' Foo Int

declareLensesFor :: [(String, String)] -> DecsQ -> DecsQ Source

Similar to `makeLensesFor`, but takes a declaration quote.

declareClassy :: DecsQ -> DecsQ Source

For each record in the declaration quote, make lenses and traversals for it, and create a class when the type has no arguments. All record syntax in the input will be stripped off.

e.g.

    declareClassy [d|
      data Foo = Foo { fooX, fooY :: Int }
        deriving Show
      |]

will create

    data Foo = Foo Int Int deriving Show
    class HasFoo t where
      foo :: Lens' t Foo
    instance HasFoo Foo where foo = id
    fooX, fooY :: HasFoo t => Lens' t Int

declareClassyFor :: [(String, (String, String))] -> [(String, String)] -> DecsQ -> DecsQ Source

Similar to `makeClassyFor`, but takes a declaration quote.

declarePrisms :: DecsQ -> DecsQ Source

Generate a `Prism` for each constructor of each data type.

e.g.

    declarePrisms [d|
      data Exp = Lit Int | Var String | Lambda{ bound::String, body::Exp }
      |]

will create

    data Exp = Lit Int | Var String | Lambda { bound::String, body::Exp }
    _Lit :: Prism' Exp Int
    _Var :: Prism' Exp String
    _Lambda :: Prism' Exp (String, Exp)

declareWrapped :: DecsQ -> DecsQ Source

Build `Wrapped` instance for each newtype.

declareFields :: DecsQ -> DecsQ Source

     declareFields = declareLensesWith defaultFieldRules

# Configuring Lenses

makeLensesWith :: LensRules -> Name -> DecsQ Source

Build lenses with a custom configuration.

declareLensesWith :: LensRules -> DecsQ -> DecsQ Source

Declare lenses for each records in the given declarations, using the specified `LensRules`. Any record syntax in the input will be stripped off.

defaultFieldRules :: LensRules Source

camelCaseFields :: LensRules Source

Field rules for fields in the form `prefixFieldname or _prefixFieldname` If you want all fields to be lensed, then there is no reason to use an `_` before the prefix. If any of the record fields leads with an `_` then it is assume a field without an `_` should not have a lens created.

Note: The `prefix` must be the same as the typename (with the first letter lowercased). This is a change from lens versions before lens 4.5. If you want the old behaviour, use `makeLensesWith` `abbreviatedFields`

underscoreFields :: LensRules Source

Field rules for fields in the form `_prefix_fieldname`

abbreviatedFields :: LensRules Source

Field rules fields in the form `prefixFieldname or _prefixFieldname` If you want all fields to be lensed, then there is no reason to use an `_` before the prefix. If any of the record fields leads with an `_` then it is assume a field without an `_` should not have a lens created.

Note that `prefix` may be any string of characters that are not uppercase letters. (In particular, it may be arbitrary string of lowercase letters and numbers) This is the behavior that `defaultFieldRules` had in lens 4.4 and earlier.

data LensRules Source

data DefName Source

Name to give to generated field optics.

Constructors

TopName Name | Simple top-level definiton name
MethodName Name Name | makeFields-style class name and method name

Instances

Eq DefName Source |
Ord DefName Source |
Show DefName Source |

lensRules :: LensRules Source

Rules for making fairly simple partial lenses, ignoring the special cases for isomorphisms and traversals, and not making any classes.

lensRulesFor Source

Arguments

:: [(String, String)]

(Field Name, Definition Name)

-> LensRules |

Construct a `LensRules` value for generating top-level definitions using the given map from field names to definition names.

classyRules :: LensRules Source

Rules for making lenses and traversals that precompose another `Lens`.

classyRules_ :: LensRules Source

lensField :: Lens' LensRules (Name -> [Name] -> Name -> [DefName]) Source

`Lens'` to access the convention for naming fields in our `LensRules`.

Defaults to stripping the _ off of the field name, lowercasing the name, and skipping the field if it doesn't start with an '_'. The field naming rule provides the names of all fields in the type as well as the current field. This extra generality enables field naming conventions that depend on the full set of names in a type.

The field naming rule has access to the type name, the names of all the field of that type (including the field being named), and the name of the field being named.

TypeName -> FieldNames -> FieldName -> DefinitionNames

lensClass :: Lens' LensRules (Name -> Maybe (Name, Name)) Source

Retrieve options such as the name of the class and method to put in it to build a class around monomorphic data types. Classy lenses are generated when this naming convention is provided. TypeName -> Maybe (ClassName, MainMethodName)

simpleLenses :: Lens' LensRules Bool Source

Generate "simple" optics even when type-changing optics are possible. (e.g. `Lens'` instead of `Lens`)

createClass :: Lens' LensRules Bool Source

Create the class if the constructor is `Simple` and the `lensClass` rule matches.

generateSignatures :: Lens' LensRules Bool Source

Indicate whether or not to supply the signatures for the generated lenses.

Disabling this can be useful if you want to provide a more restricted type signature or if you want to supply hand-written haddocks.

generateUpdateableOptics :: Lens' LensRules Bool Source

Generate "updateable" optics when `True`. When `False`, `Fold`s will be generated instead of `Traversal`s and `Getter`s will be generated instead of `Lens`es. This mode is intended to be used for types with invariants which must be maintained by "smart" constructors.

generateLazyPatterns :: Lens' LensRules Bool Source

Generate optics using lazy pattern matches. This can allow fields of an undefined value to be initialized with lenses:

    data Foo = Foo {_x :: Int, _y :: Bool}
      deriving Show

    makeLensesWith (lensRules & generateLazyPatterns .~ True) ''Foo

    > undefined & x .~ 8 & y .~ True
    Foo {_x = 8, _y = True}

The downside of this flag is that it can lead to space-leaks and code-size/compile-time increases when generated for large records. By default this flag is turned off, and strict optics are generated.

When using lazy optics the strict optic can be recovered by composing with `$!`:

    strictOptic = ($!) . lazyOptic