Fewer constraints in functions

[newhoggy]

Changelog

- description: |
    Fewer constraints in functions
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - improvement    # QoL changes e.g. refactoring
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• The change log section in the PR description has been filled in
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • round trip tests
  • integration tests
    See Running tests for more details
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-8.10.7 and ghc-9.2.7
• Self-reviewed the diff

[smelc]

How do you know that it is valid to add such a constraint here? Is this something you infer from the ledger?

[newhoggy]

If you can add and the the constraints function still compiles, in this case maryEraOnwardsConstraints, then adding the constraint is valid.

This constraint was added because code in MaryEraOnwards needed the constraint.

[smelc]

@newhoggy> And the maryEraOnwardsConstraints obtains the constraints in context thanks to the various imports right?

So in the end, this list of constraints here show what we actually need to use (as opposed to the full set of constraints if we were importing everything available)?

[newhoggy]

maryEraOnwardsConstraints obtains the constraints by case matching on the GADT. The case match tells the compiler what the specific era that is being used is after which the compiler can figure out the constraints.

The imports don't play into it except to allow use to write the XConstraints type and the Constraints type tells the compiler which of those constraints it knows about should be captured for the continuation.