GHC.Stack

Access to GHC's call-stack simulation

Since: 4.5.0.0

Documentation

errorWithStackTrace :: String -> a

Deprecated: error appends the call stack now

Like the function error, but appends a stack trace to the error message if one is available.

Since: 4.7.0.0

Profiling call stacks

currentCallStack :: IO [String]

Returns a [String] representing the current call stack. This can be useful for debugging.

The implementation uses the call-stack simulation maintained by the profiler, so it only works if the program was compiled with -prof and contains suitable SCC annotations (e.g. by using -fprof-auto). Otherwise, the list returned is likely to be empty or uninformative.

Since: 4.5.0.0

whoCreated :: a -> IO [String]

Get the stack trace attached to an object.

Since: 4.5.0.0

HasCallStack call stacks

data 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

errorWithCallStack :: HasCallStack => String -> a

as a variant of error that will get its call-site. We can access the call-stack inside errorWithCallStack with callStack.

errorWithCallStack :: HasCallStack => String -> a
errorWithCallStack msg = error (msg ++ "n" ++ prettyCallStack callStack)

Thus, if we call errorWithCallStack we will get a formatted call-stack alongside our error message.

>>> errorWithCallStack "die"
*** Exception: die
CallStack (from HasCallStack):
  errorWithCallStack, called at <interactive>:2:1 in interactive:Ghci1

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.

Since: 4.8.1.0

Instances

IsList CallStack

Be aware that 'fromList . toList = id' only for unfrozen CallStacks, since toList removes frozenness information.

Since: 4.9.0.0

Associated Types

type Item CallStack :: *

Methods

fromList :: [Item CallStack] -> CallStack

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

toList :: CallStack -> [Item CallStack]

Show CallStack

Since: 4.9.0.0

Methods

showsPrec :: Int -> CallStack -> ShowS

show :: CallStack -> String

showList :: [CallStack] -> ShowS

type Item CallStack

type Item CallStack = (String, SrcLoc)

type HasCallStack = ?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.

Since: 4.9.0.0

callStack :: HasCallStack => CallStack

Return the current CallStack.

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

Since: 4.9.0.0

emptyCallStack :: CallStack

The empty CallStack.

Since: 4.9.0.0

freezeCallStack :: CallStack -> CallStack

Freeze a call-stack, preventing any further call-sites from being appended.

pushCallStack callSite (freezeCallStack callStack) = freezeCallStack callStack

Since: 4.9.0.0

fromCallSiteList :: [([Char], SrcLoc)] -> CallStack

Convert a list of call-sites to a CallStack.

Since: 4.9.0.0

getCallStack :: CallStack -> [([Char], SrcLoc)]

Extract a list of call-sites from the CallStack.

The list is ordered by most recent call.

Since: 4.8.1.0

popCallStack :: CallStack -> CallStack

Pop the most recent call-site off the CallStack.

This function, like pushCallStack, has no effect on a frozen CallStack.

Since: 4.9.0.0

prettyCallStack :: CallStack -> String

Pretty print a CallStack.

Since: 4.9.0.0

pushCallStack :: ([Char], SrcLoc) -> CallStack -> CallStack

Push a call-site onto the stack.

This function has no effect on a frozen CallStack.

Since: 4.9.0.0

withFrozenCallStack :: HasCallStack => (HasCallStack => a) -> a

Perform some computation without adding new entries to the CallStack.

Since: 4.9.0.0

Source locations

data SrcLoc

A single location in the source code.

Since: 4.8.1.0

Constructors

SrcLoc

Fields

• srcLocPackage :: [Char]
• srcLocModule :: [Char]
• srcLocFile :: [Char]
• srcLocStartLine :: Int
• srcLocStartCol :: Int
• srcLocEndLine :: Int
• srcLocEndCol :: Int

Instances

Eq SrcLoc

Methods

(==) :: SrcLoc -> SrcLoc -> Bool

(/=) :: SrcLoc -> SrcLoc -> Bool

Show SrcLoc

Methods

showsPrec :: Int -> SrcLoc -> ShowS

show :: SrcLoc -> String

showList :: [SrcLoc] -> ShowS

prettySrcLoc :: SrcLoc -> String

Pretty print a SrcLoc.

Since: 4.9.0.0

Internals

data CostCentreStack

data CostCentre

getCurrentCCS :: dummy -> IO (Ptr CostCentreStack)

getCCSOf :: a -> IO (Ptr CostCentreStack)

clearCCS :: IO a -> IO a

ccsCC :: Ptr CostCentreStack -> IO (Ptr CostCentre)

ccsParent :: Ptr CostCentreStack -> IO (Ptr CostCentreStack)

ccLabel :: Ptr CostCentre -> IO CString

ccModule :: Ptr CostCentre -> IO CString

ccSrcSpan :: Ptr CostCentre -> IO CString

ccsToStrings :: Ptr CostCentreStack -> IO [String]

renderStack :: [String] -> String