Adding more HasCallStack s

[toyboot4e]

I suggest adding more HasCallStack constraints.

1. Missing HasCallStack constraints

Some functions in non- Generic modules are missing HasCallStack constraints.

Example: VU.(!)

VU.(!) does not have HasCallStack constraint and we don't get full trace:

Example

{- stack script --resolver lts-22.24 --package vector -}

import Data.Vector.Unboxed qualified as VU

main :: IO ()
main = do
  let !_ = (VU.fromList [0 :: Int]) VU.! 42
  return ()

The call stack does not contain VU.(!) call from main:

$ stack example-u.hs
example-u.hs: index out of bounds (42,1)
CallStack (from HasCallStack):
  error, called at src/Data/Vector/Internal/Check.hs:103:12 in vector-0.13.1.0-Aqc2YUE1Egs5WmXVMbcJ5T:Data.Vector.Internal.Check
  checkError, called at src/Data/Vector/Internal/Check.hs:109:17 in vector-0.13.1.0-Aqc2YUE1Egs5WmXVMbcJ5T:Data.Vector.Internal.Check
  check, called at src/Data/Vector/Internal/Check.hs:122:5 in vector-0.13.1.0-Aqc2YUE1Egs5WmXVMbcJ5T:Data.Vector.Internal.Check
  checkIndex, called at src/Data/Vector/Generic.hs:235:11 in vector-0.13.1.0-Aqc2YUE1Egs5WmXVMbcJ5T:Data.Vector.Generic
  !, called at src/Data/Vector/Unboxed.hs:297:7 in vector-0.13.1.0-Aqc2YUE1Egs5WmXVMbcJ5T:Data.Vector.Unboxed

VG.(!) has HasCallStack constraint and we get a full trace:

Example

{- stack script --resolver lts-22.24 --package vector -}

import Data.Vector.Unboxed qualified as VU
+import Data.Vector.Generic qualified as VG

main :: IO ()
main = do
-  let !_ = (VU.fromList [0 :: Int]) VU.! 42
+  let !_ = (VU.fromList [0 :: Int]) VG.! 42
  return ()

The call stack contains VG.(!) call from main:

$ stack example-g.hs
example-g.hs: index out of bounds (42,1)
CallStack (from HasCallStack):
  error, called at src/Data/Vector/Internal/Check.hs:103:12 in vector-0.13.1.0-Aqc2YUE1Egs5WmXVMbcJ5T:Data.Vector.Internal.Check
  checkError, called at src/Data/Vector/Internal/Check.hs:109:17 in vector-0.13.1.0-Aqc2YUE1Egs5WmXVMbcJ5T:Data.Vector.Internal.Check
  check, called at src/Data/Vector/Internal/Check.hs:122:5 in vector-0.13.1.0-Aqc2YUE1Egs5WmXVMbcJ5T:Data.Vector.Internal.Check
  checkIndex, called at src/Data/Vector/Generic.hs:235:11 in vector-0.13.1.0-Aqc2YUE1Egs5WmXVMbcJ5T:Data.Vector.Generic
  !, called at /home/tbm/dev/hs/tmp/example-g.hs:8:37 in main:Main

2. head and more

I personally like to add HasCallStack to anywhere possible.

For example, the head function does not have HasCallStack constraint. But the list's head function in base doesn't have it either. What do you think? Thank you.

[toyboot4e]

Oh no, it was already discussed A LOT in the Haskell community (outside of the vector repository). Closing as the impact seems bigger than I expected.

[Shimuuar]

Could you provide links to discussions? Adding HasCallStack to functions which could fail with out of range exception seems quite sensible to me.

[toyboot4e]

Thank you, I rushed. Let me share the links.

---

Past PR:

• Remove CPP driven error reporting in favor of HasCallStack #397

Compilation time, memory usage, runtime performance:

• Inlining functions of Unboxed vector consumes a lot of memory #274

Related:

• Adding HasCallStack to the methods of the Ix typeclass core-libraries-committee#115

---

My notes:

• HasCallStack was not added to Data.Ix. It I guess is because we'll get global backtrace mechanism (based on HasCallStack) that doesn't need user code change in the future.

• Adding HasCallStack to U.(!) etc. would be ok if the runtime/compile time cost is little.

• Adding HasCallStack to head etc. depends on policies.

• For now, we can use generic functions such as VG.(!) or VGM.write for getting backtraces. We can also define total functions such as headMaybe in the user land.