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GHC 10.0.0.20260917 · lts/ghc-10.0.x · fc4e598 · 2026-10-11

Moduleeffectful-core-2.7.1.2GHC2021

Effectful.Error.Dynamic

The dynamically dispatched variant of the Error effect.

Note: unless you plan to change interpretations at runtime or you need the Control.Monad.Except.MonadError instance for compatibility with existing code, it's recommended to use the statically dispatched variant, i.e. Effectful.Error.Static.

All caveats described in Effectful.Error.Static (in particular the interaction with threads) apply.

  • 3 types
  • 15 values

Effect

1 declaration

Handlers

Operations

valuethrowErrorWith
  1. :: (HasCallStack, Error e :> es)
  2. => (e -> String)

    The display function.

  3. -> e

    The error.

  4. -> Eff es a
#

Throw an error of type e and specify a display function in case a third-party code catches the internal exception and shows it.

valuerethrowErrorWith
  1. :: Error e :> es
  2. => (e -> String)

    The display function.

  3. -> CallStack

    The CallStack to attach to the error.

  4. -> e

    The error.

  5. -> Eff es a
#

Throw an error of type e with the given CallStack and specify a display function in case a third-party code catches the internal exception and shows it.

Useful e.g. when you want to catch an error and rethrow it converted to a different type without losing the original CallStack.

Re-exports

4 declarations
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