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GHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05

Moduleeffectful-core-2.3.0.1Haskell2010

Effectful.NonDet

Provider of the Alternative and MonadPlus instance for Eff.

  • 4 types
  • 1 class
  • 5 values

Effect

2 declarations
datadata OnEmptyPolicy
#

Policy of dealing with modifications to thread local state in the environment in branches that end up calling the Empty operation.

Note: OnEmptyKeep is significantly faster as there is no need to back up the environment on each call to :<|>:.

Constructors

Instances5Eq, Ord, Show, Generic, Rep

Handlers

Utils

2 declarations

Re-exports

5 declarations
classclass Applicative f => Alternative (f :: Type -> Type) where
#

A monoid on applicative functors.

If defined, some and many should be the least solutions of the equations:

Examples
Example1 expression
Nothing <|> Just 42Just 42
Example1 expression
[1, 2] <|> [3, 4][1,2,3,4]
Example1 expression
empty <|> print (2^15)32768

Methods

  • empty :: f a

    The identity of <|>

    empty <|> a     == a
a     <|> empty == a
  • (<|>) :: f a -> f a -> f ainfixl 3

    An associative binary operation

  • some :: f a -> f [a]

    One or more.

    Examples
    Example1 expression
    some (putStr "la")lalalalalalalalala... * goes on forever *
    Example1 expression
    some Nothingnothing
    Example1 expression
    take 5 <$> some (Just 1)* hangs forever *

    Note that this function can be used with Parsers based on Applicatives. In that case some parser will attempt to parse parser one or more times until it fails.

  • many :: f a -> f [a]

    Zero or more.

    Examples
    Example1 expression
    many (putStr "la")lalalalalalalalala... * goes on forever *
    Example1 expression
    many NothingJust []
    Example1 expression
    take 5 <$> many (Just 1)* hangs forever *

    Note that this function can be used with Parsers based on Applicatives. In that case many parser will attempt to parse parser zero or more times until it fails.

Instances45Alternative, …
typetype HasCallStack = IP "callStack" CallStack
#

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.

datadata CallStack
#

CallStacks 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.

Example1 expression
:{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.

Example1 expression
putStrLnWithCallStack "hello"helloCallStack (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.

CallStacks 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.

Instances4IsList, Show, NFData, Item