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

Since: 4.7.0.0

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

Get the stack trace attached to an object.

Since: 4.5.0.0

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

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

Return the current CallStack.

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

Since: 4.9.0.0

The empty CallStack.

Since: 4.9.0.0

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

pushCallStack callSite (freezeCallStack callStack) = freezeCallStack callStack

Since: 4.9.0.0

Convert a list of call-sites to a CallStack.

Since: 4.9.0.0

Extract a list of call-sites from the CallStack.

The list is ordered by most recent call.

Since: 4.8.1.0

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

Pretty print a CallStack.

Since: 4.9.0.0

Push a call-site onto the stack.

This function has no effect on a frozen CallStack.

Since: 4.9.0.0

Perform some computation without adding new entries to the CallStack.

Since: 4.9.0.0

A single location in the source code.

Since: 4.8.1.0

Pretty print a SrcLoc.

Since: 4.9.0.0