Re: How do I make a constraint tuple in Core?

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 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 wrote:

...

How about `tc_tuple`?

On Tue, Jun 19, 2018 at 2:53 PM, Ryan Scott 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... [2]

http://git.haskell.org/ghc.git/blob/676c5754e3f9e1beeb5f01e0265ffbdc0e6f49e9... [3]

http://git.haskell.org/ghc.git/blob/676c5754e3f9e1beeb5f01e0265ffbdc0e6f49e9...

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