[Haskell-cafe] HasCallStack - runtime costs?

[Johannes Waldmann]

Thanks, Eric.


> If you import a function with a HasCallStack constraint
> there's no way to disable the overhead

But then this means that each library
should avoid these constraints?

Or not, because this rightly scares away users
from calling nontotal functions?


> ... with a couple special rules for
> building dictionaries in GHC's constraint solver.

then activation of these special rules could be a compiler switch?
Though it'd probably create a mess with separate compilation/linking.
But switching via CPP will do the same.


> class Semiring s where
>   plus :: HasCallStack => s -> s -> s
>   times :: HasCallStack => s -> s -> s
> I'm curious, why do plus and times take a CallStack?

because I want to (be able to) debug implementations.

simplified example:

{-# language ConstrainedClassMethods #-}

import GHC.Stack.Types

class C a where
  p ::                 a -> Int
  q :: HasCallStack => a -> Int

instance C () where
  p x = error "huh"
  q x = error "huh"


Calling  q  gives more information:


*Main> p ()
*** Exception: huh
CallStack (from HasCallStack):
  error, called at CS.hs:10:9 in main:Main

*Main> q ()
*** Exception: huh
CallStack (from HasCallStack):
  error, called at CS.hs:11:9 in main:Main
  q, called at <interactive>:44:1 in interactive:Ghci1
  it, called at <interactive>:44:1 in interactive:Ghci1


I did not see another way than to change the type
of the method in the class. (Which looks terrible of course.)


- J.