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1. currentCallStack :: IO [String]

   base	GHC.Stack.CCS

   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.

2. data CallStack

   base	GHC.Stack.Types

   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.

3. EmptyCallStack :: CallStack

   base	GHC.Stack.Types

   No documentation available.

4. FreezeCallStack :: CallStack -> CallStack

   base	GHC.Stack.Types

   Freeze the stack at the given CallStack, preventing any further call-sites from being pushed onto it.

5. PushCallStack :: [Char] -> SrcLoc -> CallStack -> CallStack

   base	GHC.Stack.Types

   No documentation available.

6. emptyCallStack :: CallStack

   base	GHC.Stack.Types

   The empty CallStack.

7. freezeCallStack :: CallStack -> CallStack

   base	GHC.Stack.Types

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

   pushCallStack callSite (freezeCallStack callStack) = freezeCallStack callStack
   

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

   base	GHC.Stack.Types

   Convert a list of call-sites to a CallStack.

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

   base	GHC.Stack.Types

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

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

    base	GHC.Stack.Types

    Push a call-site onto the stack. This function has no effect on a frozen CallStack.

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