6.19.5. HasCallStack

GHC.Stack.HasCallStack is 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 and access it as a Haskell value by using callStack.

One can then use functions from GHC.Stack to inspect or pretty print (as is done in f below) the call stack.

f :: HasCallStack => IO ()
f = putStrLn (prettyCallStack callStack)

g :: HasCallStack => IO ()
g = f

Evaluating f directly shows a call stack with a single entry, while evaluating g, which also requests its call-site, shows two entries, one for each computation “annotated” with HasCallStack.

ghci> f
CallStack (from HasCallStack):
  f, called at <interactive>:19:1 in interactive:Ghci1
ghci> g
CallStack (from HasCallStack):
  f, called at <interactive>:17:5 in main:Main
  g, called at <interactive>:20:1 in interactive:Ghci2

The error function from the Prelude supports printing the call stack that led to the error in addition to the usual error message:

ghci> error "bad"
*** Exception: bad
CallStack (from HasCallStack):
  error, called at <interactive>:25:1 in interactive:Ghci5

The call stack here consists of a single entry, pinpointing the source of the call to error. However, by annotating several computations with HasCallStack, figuring out the exact circumstances and sequences of calls that lead to a call to error becomes a lot easier, as demonstrated with the simple example below.

f :: HasCallStack => IO ()
f = error "bad bad bad"

g :: HasCallStack => IO ()
g = f

h :: HasCallStack => IO ()
h = g

ghci> h
*** Exception: bad bad bad
CallStack (from HasCallStack):
  error, called at call-stack.hs:4:5 in main:Main
  f, called at call-stack.hs:7:5 in main:Main
  g, called at call-stack.hs:10:5 in main:Main
  h, called at <interactive>:28:1 in interactive:Ghci1

The CallStack will only extend as far as the types allow it, for example

myHead :: HasCallStack => [a] -> a
myHead []     = error "empty"
myHead (x:xs) = x

bad :: Int
bad = myHead []

ghci> bad
*** Exception: empty
CallStack (from HasCallStack):
  error, called at Bad.hs:8:15 in main:Bad
  myHead, called at Bad.hs:12:7 in main:Bad

includes the call-site of error in myHead, and of myHead in bad, but not the call-site of bad at the GHCi prompt.

GHC solves HasCallStack constraints in two steps:

  1. If there is a CallStack in scope – i.e. the enclosing definition has a HasCallStack constraint – GHC will push the new call-site onto the existing CallStack.
  2. Otherwise GHC will solve the HasCallStack constraint for the singleton CallStack containing just the current call-site.

Importantly, GHC will never infer a HasCallStack constraint, you must request it explicitly.

CallStack is kept abstract, but GHC provides a function

getCallStack :: CallStack -> [(String, SrcLoc)]

to access the individual call-sites in the stack. The String is the name of the function that was called, and the SrcLoc provides the package, module, and file name, as well as the line and column numbers.

GHC.Stack additionally exports a function withFrozenCallStack that allows users to freeze the current CallStack, preventing any future push operations from having an effect. This can be used by library authors to prevent CallStacks from exposing unnecessary implementation details. Consider the myHead example above, the error line in the printed stack is not particularly enlightening, so we might choose to suppress it by freezing the CallStack that we pass to error.

myHead :: HasCallStack => [a] -> a
myHead []     = withFrozenCallStack (error "empty")
myHead (x:xs) = x

ghci> myHead []
*** Exception: empty
CallStack (from HasCallStack):
  myHead, called at Bad.hs:12:7 in main:Bad

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.

6.19.5.1. Compared with other sources of stack traces

HasCallStack does not interact with the RTS and does not require compilation with -prof. On the other hand, as the CallStack is built up explicitly via the HasCallStack constraints, it will generally not contain as much information as the simulated call-stacks maintained by the RTS.