Documentation

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>: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: base-4.8.1.0

Return the current CallStack.

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

Since: base-4.9.0.0

Pretty print a SrcLoc.

Since: base-4.9.0.0

Convert a list of call-sites to a CallStack.

Since: base-4.9.0.0

Extract a list of call-sites from the CallStack.

The list is ordered by most recent call.

Since: base-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: base-4.9.0.0

A single location in the source code.

Since: base-4.8.1.0

Type of a filter for CallSite

Type of an entry in a CallStack

Type of a column of a SrcLoc

Type of a line number of a SrcLoc

Type of a filename of a SrcLoc | e.g. `srcfullAgdaUtilsFoo.hs`

Type of the name of a function in a CallSite | e.g. proveEverything

Type of the module name of a SrcLoc | e.g. Foo

Type of the package name of a SrcLoc | e.g. `Agda-2.…`

The same as the un-exported internal function in GHC.Exceptions (prettyCallStackLines) Prints like: doFoo, called at foo.hs:190:24 in main:Main

Pretty-print a CallStack. This has a few differences from GHC.Stack.prettyCallStackLines. We omit the "CallStack (from GetCallStack)" header line for brevity. If there is only one entry (which is common, due to the manual nature of the HasCallStack constraint), shows the entry on one line. If there are multiple, then the following lines are indented.

Get the most recent CallSite in a CallStack, if there is one.

CallStack comprising only the most recent CallSite

Transform a CallStack by transforming its list of CallSite

Transform a CallStack by filtering each CallSite

Pops n entries off a CallStack using popCallStack. Note that frozen callstacks are unaffected.