GHC.Stack

Copyright	(c) The University of Glasgow 2011
License	see libraries/base/LICENSE
Maintainer	cvs-ghc@haskell.org
Stability	internal
Portability	non-portable (GHC Extensions)
Safe Haskell	Trustworthy
Language	Haskell2010

errorWithStackTrace :: String -> a Source

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] Source

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

The implementation uses the call-stack simulation maintined 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] Source

Get the stack trace attached to an object.

Since: 4.5.0.0

HasCallStack call stacks

data CallStack Source

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

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: 4.9.0.0

callStack :: HasCallStack => CallStack Source

Return the current CallStack.

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

Since: 4.9.0.0

emptyCallStack :: CallStack Source

The empty CallStack.

Since: 4.9.0.0

freezeCallStack :: CallStack -> CallStack Source

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 Source

Convert a list of call-sites to a CallStack.

Since: 4.9.0.0

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

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 Source

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 Source

Pretty print a CallStack.

Since: 4.9.0.0

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

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 Source

Perform some computation without adding new entries to the CallStack.

Since: 4.9.0.0

Source locations

data SrcLoc Source

A single location in the source code.

Since: 4.8.1.0

prettySrcLoc :: SrcLoc -> String Source

Pretty print a SrcLoc.

Since: 4.9.0.0

Internals

data CostCentreStack Source

data CostCentre Source

getCurrentCCS :: dummy -> IO (Ptr CostCentreStack) Source

getCCSOf :: a -> IO (Ptr CostCentreStack) Source

clearCCS :: IO a -> IO a Source

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

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

ccLabel :: Ptr CostCentre -> IO CString Source

ccModule :: Ptr CostCentre -> IO CString Source

ccSrcSpan :: Ptr CostCentre -> IO CString Source

ccsToStrings :: Ptr CostCentreStack -> IO [String] Source

renderStack :: [String] -> String Source