How do I make a constraint tuple in Core?

[Ryan Scott]

Unfortunately, I can't directly use tc_tuple, since I don't have access to
the Haskell AST forms I need to make that work (I'm constructing everything
directly in Core). On the other hand, the implementation of tc_tuple does
have one nugget of wisdom in that it reveals how GHC creates a constraint
tuple *type constructor*. Namely, `tcLookupTyCon (cTupleTyConName arity)`
for some `arity`.

That's still a bit inconvenient, as `tcLookupTyCon` forces me to work in a
monadic context (whereas the code I've been working on has been pure up to
this point). Is there not a pure way to retrieve a constraint tuple type
constructor?

Ryan S.

On Tue, Jun 19, 2018 at 10:07 AM Matthew Pickering <
matthewtpickering at gmail.com> wrote:

> How about `tc_tuple`?
>
> On Tue, Jun 19, 2018 at 2:53 PM, Ryan Scott <ryan.gl.scott at gmail.com>
> wrote:
> > I'm currently working on some code in which I need to produce a Core Type
> > that mentions a constraint tuple. I thought that there must surely exist
> > some way to construct a constraint tuple using the GHC API, but to my
> > astonishment, I could not find anything. The closest thing I found was
> > mk_tuple [1], which gives you the ability to make boxed and unboxed
> tuples,
> > but not constraint tuples.
> >
> > I then thought to myself, "But wait, PartialTypeSignatures has to create
> > constraint tuples, right? How does that part of the code work?" To my
> > horror, I discovered that PartialTypeSignatures actually creates *boxed*
> > tuples (see mk_ctuple here [2]), then hackily treats them as constraint
> > tuples, as explained in Note [Extra-constraint holes in partial type
> > signatures] [3]. I tried reading that Note, but I couldn't follow the
> > details.
> >
> > Is there a simpler way to create a constraint tuple that I'm not aware
> of?
> >
> > Ryan S.
> > -----
> > [1]
> >
> http://git.haskell.org/ghc.git/blob/676c5754e3f9e1beeb5f01e0265ffbdc0e6f49e9:/compiler/prelude/TysWiredIn.hs#l810
> > [2]
> >
> http://git.haskell.org/ghc.git/blob/676c5754e3f9e1beeb5f01e0265ffbdc0e6f49e9:/compiler/typecheck/TcBinds.hs#l1036
> > [3]
> >
> http://git.haskell.org/ghc.git/blob/676c5754e3f9e1beeb5f01e0265ffbdc0e6f49e9:/compiler/typecheck/TcHsType.hs#l2367
> >
> > _______________________________________________
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> >
>
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