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warning-for-head-and-tail.md

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What

CLC has approved the proposal to add {-# WARNING #-} to Data.List.{head,tail}. This means that the existing users of these functions will receive a compile-time warning about their partiality.

When

The change is scheduled for GHC 9.8.

How

This is not a breaking change, unless a package forces -Werror.

If you don't want to change any Haskell code and want to carry on with using Data.List.{head,tail}:

  1. If you use -Werror, set -Wwarn=x-partial to downgrade this area of compiler messages from errors to warnings, so that your build can succeed. Note that x-partial is a new group of warnings, introduced in GHC 9.8, so you might need to additionally set -Wno-unrecognised-warning-flags or -Wwarn=unrecognised-warning-flags to maintain backward compatibility with older GHC releases.

  2. If you are annoyed by warnings, set -Wno-x-partial to quash this area of compiler messages. Use -Wno-unrecognised-warning-flags to maintain backward compatibility with older GHC releases. To disable the warnings once and forever in GHCi, put :set -Wno-x-partial -Wno-unrecognised-warning-flags into ~/.ghci config file. Beware that this also disables any other warnings in the custom -x-partial warning group!

If you are happy to change code to eliminate warning messages:

  1. The easiest way to migrate away from tail is to replace it with drop 1. This is a drop-in replacement unless you are relying on tail's partiality.

  2. Instead of head you can use explicit pattern matching: replace any head foo with

    case foo of
  [] -> error "<explain, why it is impossible to encounter an empty list here>"
  hd : _ -> hd

  3. Another option is to use Data.List.uncons or Data.Maybe.listToMaybe: replace any head foo with

    maybe (error "...") fst (uncons foo)

    or

    fromMaybe (error "...") (listToMaybe foo)

  4. It is even better to enforce non-emptiness of the list in types, using Data.List.NonEmpty. This might however require non-local refactoring.

  5. If the list is guaranteed to be not only non-empty, but actually infinite, one can be benefit from enforcing this invariant in types as well. E. g., use Stream, streams or infinite-list.

  6. As an ultimate measure you can hide Prelude.{head,tail} and define your own in a utility module, or use headErr and tailErr from Safe.

PR template

Here is a template, which you can use when raising PRs against affected libraries.

Title: Avoid Data.List.{head,tail}

CLC has approved the proposal to add {-# WARNING #-} to Data.List.{head,tail} (#87). It means that usage of head and tail will emit compile-time warnings starting from GHC 9.8.

Migration guide and more info: https://github.com/haskell/core-libraries-committee/blob/main/guides/warning-for-head-and-tail.md