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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