HORIZON HASKELLDocslts/ghc-9.10.x248f8f02026-10-05Search names, modules, packages, or :: a typeCtrl K

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

Moduleeffectful-core-2.3.0.1Haskell2010

Effectful.Error.Static

Support for handling errors of a particular type, i.e. checked exceptions.

The Error effect is not a general mechanism for handling regular exceptions, that's what functions from the exceptions library are for (see Control.Monad.Catch for more information).

In particular, regular exceptions of type e are distinct from errors of type e and will not be caught by functions from this module:

Example1 expression
import qualified Control.Monad.Catch as E
Example1 expression
boom = error "BOOM!"
Example1 expression
runEff . runError @ErrorCall $ boom `catchError` \_ (_::ErrorCall) -> pure "caught"*** Exception: BOOM!...

If you want to catch regular exceptions, you should use catch (or a similar function):

Example1 expression
runEff $ boom `E.catch` \(_::ErrorCall) -> pure "caught""caught"

On the other hand, functions for safe finalization and management of resources such as finally and bracket work as expected:

Example1 expression
msg = liftIO . putStrLn
Example1 expression
:{runEff . runErrorNoCallStack @String $ do  E.bracket_ (msg "Beginning.")             (msg "Cleaning up.")             (msg "Computing." >> throwError "oops" >> msg "More."):}Beginning.Computing.Cleaning up.Left "oops"

Note: unlike the ExceptT monad transformer from the transformers library, the order in which you handle the Error effect with regard to other stateful effects does not matter. Consider the following:

Example2 expressions
import qualified Control.Monad.State.Strict as Timport qualified Control.Monad.Except as T
Example1 expression
m1 = (T.modify (++ " there!") >> T.throwError "oops") `T.catchError` \_ -> pure ()
Example1 expression
(`T.runStateT` "Hi") . T.runExceptT $ m1(Right (),"Hi there!")
Example1 expression
T.runExceptT . (`T.runStateT` "Hi") $ m1Right ((),"Hi")

Here, whether state updates within the catchError block are discarded or not depends on the shape of the monad transformer stack, which is surprising and can be a source of subtle bugs. On the other hand:

Example1 expression
import Effectful.State.Static.Local
Example1 expression
m2 = (modify (++ " there!") >> throwError "oops") `catchError` \_ (_::String) -> pure ()
Example1 expression
runEff . runState "Hi" . runError @String $ m2(Right (),"Hi there!")
Example1 expression
runEff . runError @String . runState "Hi" $ m2Right ((),"Hi there!")

Here, no matter the order of effects, state updates made within the catchError block before the error happens always persist, giving predictable behavior.

Hint: if you'd like to reproduce the transactional behavior with the State effect, appropriate usage of bracketOnError will do the trick.

  • 3 types
  • 10 values

Effect

1 declaration
datadata Error e (a :: Type -> Type) b
#

Provide the ability to handle errors of type e.

Instances2DispatchOf, StaticRep

Handlers

Operations

valuecatchError
  1. :: Error e :> es
  2. => Eff es a

    The inner computation.

  3. -> (CallStack -> e -> Eff es a)

    A handler for errors in the inner computation.

  4. -> Eff es a
#

Handle an error of type e.

valuehandleError
  1. :: Error e :> es
  2. => (CallStack -> e -> Eff es a)

    A handler for errors in the inner computation.

  3. -> Eff es a

    The inner computation.

  4. -> Eff es a
#

The same as flip catchError, which is useful in situations where the code for the handler is shorter.

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