Deprecation of Optional/fold and Optional/build

Migrate your code to remove the Optional/fold and Optional/build builtins

On December 22, 2019 the language standard changed to support a new way of destructuring Optional values using the existing merge keyword, and we proposed to remove the now-redundant Optional/fold builtin . Optional/build is removed in sync with Optional/fold, since it was defined only for consistency with other builtin folds.



Instead of destructuring an Optional like this:

Optional/fold Bool (Some True) Bool (λ(b : Bool)  b) False

… users should use merge like this:

merge { None = False, Some = λ(b : Bool)  b } (Some True)


Users of Optional/build should simply use Some and None directly.


Optional/build and Optional/fold will be phased out in three steps:

  • Phase 1 - Extend merge to work on Optionals

    • Standard version: 13.0.0

    • Haskell implementation version: 1.29.0

    • Ruby implementation version: To be determined

    • Rust implementation version: 0.3.0

    The first phase is backwards compatible, meaning that both Optional/fold and merge are supported.

  • Phase 2 - Remove use of the Optional/fold and Optional/build builtins in the Prelude

    Prelude.Optional.fold, and other Prelude functions will be changed to use merge instead of the deprecated builtins.

    • Standard version: 15.0.0

  • Phase 3 - Remove the Optional/fold and Optional/fold builtins

    • Standard version: 17.0.0

    • Haskell implementation version: 1.33.0

    • Ruby implementation version: To be determined

    • Rust implementation version: 0.6.0

    This change is strongly backwards incompatible by removing support for the old List-like syntax for Optional literals, breaking all code that still uses the old syntax.


  • Phase 1 + 2 - Automatically migrate your code

    Starting in dhall-1.30.0, dhall lint will automatically migrate code using the deprecated builtins to ease the migration process. This is a safe and behavior-preserving transformation.

  • Phase 3 - Your code breaks if you haven’t migrated

    During Phase 3 the deprecation cycle is complete and if you haven’t migrated then your code will fail to type-check since the Optional/fold and Optional/build builtins will be out of scope.