Hehe thank you (again) to the authors of Hugs. The answer seems to be in exactly one place -- it just took a long time to make sure there weren't other places interfering.
type.c routine typeFreshPat(l,p) /* find type of pattern, assigning fresh type variables to each var */ This sets tcMode = NEW_PATTERN /* type-checking Mode */ then recurses on typeExpr(l,p) -- so tcMode is in effect an additional parameter typeExpr( ) returns `p` with suitable bindings; but also returns the induced context in global var `preds` -- updated as a side-effect by calling a slather of worker routines -- chiefly in the #include'd preds.c. If I save `preds` before recursing on typeExpr(l,p); and restore upon return; I get no context induced for a datatype constructor appearing in pattern position. The datatype context _is_ still inferred for the constructor appearing in an expression, so the constraints still apply for building. I did break something, though: numeric patterns -- that is, literals -- aren't getting recognised, so are giving pattern match fails. The Language Report 3.17.2, point 7 says matching to numeric literals should succeed if `v == k`; that still works if I move the `==` test into a guard. It works as a pattern ok for Char or String literals, so I guess I've broken how Hugs translates the numeric literals to the `fromInteger` etc equality test. Mod'ing this behaviour is giving some interesting semantics -- I'll discuss on Hugs Users list. On Tue, 5 Oct 2021 at 13:36, Anthony Clayden <anthony.d.clay...@gmail.com> wrote: > > From this discussion 'Contexts on datatype declarations' > > http://web.archive.org/web/20151208175102/http://code.haskell.org/~dons/haskell-1990-2000/threads.html#04062 > > It looks like (at least at the time) GHC had separate functions for > matching vs building using a constructor. ... > > What I can't find is where Hugs infers the type for a data constructor > appearing in matching position. > > Any hints? Thanks in advance > >
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