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Improve type variable name generation and recursive type detection when printing type errors #10488

Merged
merged 2 commits into from
Oct 15, 2021
Merged

Improve type variable name generation and recursive type detection when printing type errors #10488

merged 2 commits into from
Oct 15, 2021

Commits on Oct 12, 2021

  1. Separate type variable naming and loop marking when printing errors 

    Improve type variable name generation and recursive type detection
when printing type errors by

1. Iterating over the whole trace and reserving type variable names
   once, to ensure that type variable names aren't reused in confusing
   ways between types.

2. Detecting recursive types (i.e., marking loops) for each type
   individually during printing, ensuring that spurious `as 'a`
   clauses aren't generated and that `as`-bound names can't be
   referred to across different types.

This involved updating the `Printtyp` module: first refactoring
`mark_loops_rec` into a general case analysis
`iter_type_expr_for_printing`, and then implementing both
`reserve_names` and `mark_loops` in terms of it (via
`Names.add_named_vars` and a new `mark_loops_rec` that only handles
marking loops and not generating names).

As a result, the API for printing types also changed:

* `type_expr` still does what it always did.

* To handle multiple types simultaneously, use `prepare_for_printing`
  and `prepared_type_expr` (replacing `marked_type_expr` and the
  associated marking facilities).

* Within `Printtyp`, we use `named_type_expr`, which is in-between the
  other formatters and assumes names have been generated but does its
  own loop marking.
    @antalsz
    antalsz committed Oct 12, 2021
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  2. Respond to review for ocaml#10488 and add Changes entry

    @antalsz
    antalsz committed Oct 12, 2021
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