Hacktoberfest 2026: the issues maintainers tagged for October, open and beginner-friendly. Browse Hacktoberfest issues

Adding more `HasCallStack` s

Open
#494 3 comments 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
haskell
Domain
data

Research direction

Start by reviewing the non-Generic vector modules, especially the VU.(!) example, and compare them with Data/Vector/Generic.hs where VG.(!) already carries HasCallStack. Use the provided stack-script examples to inspect whether caller frames appear for selected functions. Done means agreeing on the functions that should gain constraints and confirming the resulting traces, including the open question about head.

Written by the indexing model from the issue text.

Description

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.

Dominant language
Haskell
Stars
401
Forks
145
No merged PRs in 30d

Contributor guide

No contributing guide indexed for this repository

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

More from haskell/vector

All issues in haskell/vector

Similar issues

More Haskell issues

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.