Stack package:ghc-internal

Access to GHC's call-stack simulation

Haskell representation of a StgStack* that was created (cloned) with a function in GHC.Stack.CloneStack. Please check the documentation in that module for more detailed explanations.

The current thread's stack exceeded its limit. Since an exception has been raised, the thread's stack will certainly be below its limit again, but the programmer should take remedial action immediately.

The state of the execution stack

Representation for the source location where a return frame was pushed on the stack. This happens every time when a case ... of scrutinee is evaluated.

A frozen snapshot of the state of an execution stack.

How many stack frames in the given StackTrace

List the frames of a stack trace.

like trace, but additionally prints a call stack if one is available. In the current GHC implementation, the call stack is only available if the program was compiled with -prof; otherwise traceStack behaves exactly like trace. Entries in the call stack correspond to SCC annotations, so it is a good idea to use -fprof-auto or -fprof-auto-calls to add SCC annotations automatically.

A variant of error that does not produce a stack trace.

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.

>>> :{
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.

>>> putStrLnWithCallStack "hello"
hello
CallStack (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.

Extract a list of call-sites from the CallStack. The list is ordered by most recent call.

collect HasCallStack backtraces

This is a module for efficient stack traces. This stack trace implementation is considered low overhead. Basic usage looks like this:

import GHC.Internal.ExecutionStack

myFunction :: IO ()
myFunction = do
putStrLn =<< showStackTrace

Your GHC must have been built with libdw support for this to work.

user@host:~$ ghc --info | grep libdw
,("RTS expects libdw",YES)

Get a trace of the current execution stack state. Returns Nothing if stack trace support isn't available on host machine.