How do I make a constraint tuple in Core?

[Matthew Pickering]

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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