haskell-gi-base-0.26.9: Foundation for libraries generated by haskell-gi

Safe Haskell | Safe-Inferred
Language | Haskell2010

Data.GI.Base.CallStack

Description

A compatibility layer for `CallStack`, so that we can have uniform signatures even in old GHC versions (even if the functionality itself does not work there).

Synopsis

- type HasCallStack = ?callStack :: CallStack
- data CallStack
- prettyCallStack :: CallStack -> String
- callStack :: HasCallStack => CallStack

# Documentation

type HasCallStack = ?callStack :: CallStack Source #

Request a CallStack.

NOTE: The implicit parameter `?callStack :: CallStack` is an implementation detail and should not be considered part of the `CallStack` API, we may decide to change the implementation in the future.

Since: base-4.9.0.0

data CallStack Source #

`CallStack`s are a lightweight method of obtaining a partial call-stack at any point in the program.

A function can request its call-site with the `HasCallStack` constraint. For example, we can define

    putStrLnWithCallStack :: HasCallStack => String -> IO ()

as a variant of `putStrLn` that will get its call-site and print it, along with the string given as argument. We can access the call-stack inside `putStrLnWithCallStack` with `callStack`.

    >>> :{
    putStrLnWithCallStack :: HasCallStack => String -> IO ()
    putStrLnWithCallStack msg = do
      putStrLn msg
      putStrLn (prettyCallStack callStack)
    :}

Thus, if we call `putStrLnWithCallStack` we will get a formatted call-stack alongside our string.

    >>> putStrLnWithCallStack "hello"
    hello
    CallStack (from HasCallStack):
      putStrLnWithCallStack, called at <interactive>:... in interactive:Ghci...

GHC solves `HasCallStack` constraints in three steps:

1. If there is a `CallStack` in scope -- i.e. the enclosing function has a `HasCallStack` constraint -- GHC will append the new call-site to the existing `CallStack`.
2. If there is no `CallStack` in scope -- e.g. in the GHCi session above -- and the enclosing definition does not have an explicit type signature, GHC will infer a `HasCallStack` constraint for the enclosing definition (subject to the monomorphism restriction).
3. If there is no `CallStack` in scope and the enclosing definition has an explicit type signature, GHC will solve the `HasCallStack` constraint for the singleton `CallStack` containing just the current call-site.

`CallStack`s do not interact with the RTS and do not require compilation with `-prof`. On the other hand, as they are built up explicitly via the `HasCallStack` constraints, they will generally not contain as much information as the simulated call-stacks maintained by the RTS.

A `CallStack` is a `[(String, SrcLoc)]`. The `String` is the name of function that was called, the `SrcLoc` is the call-site. The list is ordered with the most recently called function at the head.

NOTE: The intrepid user may notice that `HasCallStack` is just an alias for an implicit parameter `?callStack :: CallStack`. This is an implementation detail and should not be considered part of the `CallStack` API, we may decide to change the implementation in the future.

Since: base-4.8.1.0

#### Instances

Instances details

IsList CallStack | Be aware that 'fromList . toList = id' only for unfrozen `CallStack`s, since `toList` removes frozenness information. Since: base-4.9.0.0

Instance details

Defined in GHC.Internal.IsList

Associated Types

type Item CallStack |
Instance details Defined in GHC.Internal.IsList type Item CallStack = (String, SrcLoc)

Methods

fromList :: [Item CallStack] -> CallStack Source #

fromListN :: Int -> [Item CallStack] -> CallStack Source #

toList :: CallStack -> [Item CallStack] Source #

Show CallStack | Since: base-4.9.0.0
Instance details Defined in GHC.Internal.Show Methods showsPrec :: Int -> CallStack -> ShowS Source # show :: CallStack -> String Source # showList :: [CallStack] -> ShowS Source #
type Item CallStack |
Instance details Defined in GHC.Internal.IsList type Item CallStack = (String, SrcLoc)

prettyCallStack :: CallStack -> String Source #

Pretty print a `CallStack`.

Since: ghc-internal-4.9.0.0

callStack :: HasCallStack => CallStack Source #

Return the current `CallStack`.

Does *not* include the call-site of `callStack`.

Since: base-4.9.0.0