Make locally abstract value constraints structural in the parsetree - #8575
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Replace the desugared encoding of [let f: type a. t = e] - a Ptyp_poly
pattern constraint plus a Pexp_newtype chain over a Pexp_constraint,
with the type stored twice and no AST invariant ensuring that the copies
agree - with a structural field on the binding:
pvb_constraint: {pvc_newtypes: string loc list; pvc_type: core_type}
Only the [type a.] form uses the field; plain constraints and explicit
polymorphic annotations keep their existing representation. The type is
stored once, and [varify_constructors] now runs in exactly one place,
inside the type checker.
With functions already carrying their locally abstract type parameters in
Pexp_fun.newtypes, this removes the last place where the parser constructs
Pexp_newtype. Delete the constructor from the current parsetree, along with
the Texp_newtype exp_extra, which had no consumer beyond no-op iterators and
the debug printer. The CMT magic number is bumped to Caml1999T024; the CMI
format is unchanged.
Type checking follows the same design as the function case (and OCaml
5.x): type_let introduces the locally abstract types into scope via
type_newtype, types the body against the constraint, and unifies with the
pattern's polymorphic type. This preserves the semantics of the former
desugaring.
The frozen v0 PPX bridge expands the field back into the historical
wrapper-chain encoding and recognizes well-formed instances of that
encoding on the way in, verified by unit tests. A v0 Pexp_newtype chain
that cannot be represented - such as one that does not enclose ReScript's
Function$ encoding, or one whose structure was changed by a PPX - now
becomes a located ocaml.error extension with an explicit message. This is
the only intentional reduction in accepted v0 PPX output.
Formatter bug fix covered by syntax fixtures: a trailing comment between
the constraint type and [=] is no longer dropped. An end-to-end GADT test
checks that refinement still works with the new binding field.
Signed-Off-By: Cristiano Calcagno <cristianoc@users.noreply.github.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Codecov Report❌ Patch coverage is Additional details and impacted files@@ Coverage Diff @@
## codex/nary-newtypes #8575 +/- ##
=======================================================
+ Coverage 75.85% 75.93% +0.07%
=======================================================
Files 474 474
Lines 62835 62902 +67
=======================================================
+ Hits 47665 47765 +100
+ Misses 15170 15137 -33
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Part of the n-ary functions series tracked in #8557 (item 9 of 10; stacked on #8574 via base branch; each PR merges independently).
What
Replace the desugared encoding of [let f: type a. t = e] - a Ptyp_poly
pattern constraint plus a Pexp_newtype chain over a Pexp_constraint,
with the type stored twice and no AST invariant ensuring that the copies
agree - with a structural field on the binding:
pvb_constraint: {pvc_newtypes: string loc list; pvc_type: core_type}
Only the [type a.] form uses the field; plain constraints and explicit
polymorphic annotations keep their existing representation. The type is
stored once, and [varify_constructors] now runs in exactly one place,
inside the type checker.
With functions already carrying their locally abstract type parameters in
Pexp_fun.newtypes, this removes the last place where the parser constructs
Pexp_newtype. Delete the constructor from the current parsetree, along with
the Texp_newtype exp_extra, which had no consumer beyond no-op iterators and
the debug printer. The CMT magic number is bumped to Caml1999T024; the CMI
format is unchanged.
Type checking follows the same design as the function case (and OCaml
5.x): type_let introduces the locally abstract types into scope via
type_newtype, types the body against the constraint, and unifies with the
pattern's polymorphic type. This preserves the semantics of the former
desugaring.
The frozen v0 PPX bridge expands the field back into the historical
wrapper-chain encoding and recognizes well-formed instances of that
encoding on the way in, verified by unit tests. A v0 Pexp_newtype chain
that cannot be represented - such as one that does not enclose ReScript's
Function$ encoding, or one whose structure was changed by a PPX - now
becomes a located ocaml.error extension with an explicit message. This is
the only intentional reduction in accepted v0 PPX output.
Formatter bug fix covered by syntax fixtures: a trailing comment between
the constraint type and [=] is no longer dropped. An end-to-end GADT test
checks that refinement still works with the new binding field.
🤖 Generated with Claude Code