Datatype Constructors: selector/matcher vs builder 'function'

[Anthony Clayden]

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.clayden at 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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