On Sat, 22 Dec 2018 at 7:08 PM, Anthony Clayden <anthony_clayden@clear.net.nz> wrote:

On Mon, 10 Dec 2018 at 6:12 PM, Anthony Clayden <anthony_clayden@clear.net.nz> wrote:

On Mon, 10 Dec 2018 at 9:36 PM, Tom Schrijvers wrote:

Maybe our Haskell'17 paper about Elaboration on Functional Dependencies sheds some more light on your problem:

https://people.cs.kuleuven.be/~tom.schrijvers/Research/papers/haskell2017a.pdf

I feel I'm not getting much light shed. No amount of adding `(~)` in GHC nor `TypeCast`+`typeCast` method in Hugs will get #9627 function `f` to compile. So my 'specific example' earlier in this thread shows `(~)` is moderately eager/more eager than FunDeps alone, but not eager enough for #9627.

Can anybody explain @yav's comment here 

https://github.com/ghc-proposals/ghc-proposals/pull/158#issuecomment-449590613 "currently equality constraints have no run-time evidence associated with them".

Haskell has type-erasure semantics. Why would I need any _run-time_ evidence for anything to do with type improvement? What impact would the lack of evidence have at run-time?

Does this mean `(~)` constraints don't produce evidence at compile time? Which is also the difficulty for type improvement under FunDeps.

(I tried a type family to achieve the same effect as `(~)` with an injective TF. No improvement in improvement.)

AntC

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