How do I make a constraint tuple in Core?

[Matthew Pickering]

Can you be a bit more precise about what you are doing? Constructing
core like this is quite hairy.

The "tuple" part doesn't really exist in core, a constraint tuple is
curried. So foo :: (C1 a, C2 a) => ... desugars to `foo = /\ a . \
$dC1 . \$dC2 -> ...`.

Cheers,

Matt



On Tue, Jun 19, 2018 at 4:48 PM, Ryan Scott <ryan.gl.scott at gmail.com> wrote:
> 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
>> >
>> > _______________________________________________
>> > ghc-devs mailing list
>> > ghc-devs at haskell.org
>> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>> >