[Haskell-cafe] working my way through Data.Type.Equality...my head hurts...
nicholls.mark at vimn.com
Tue Jan 13 13:21:49 UTC 2015
the 1st link is a bit challenging...the 2nd I can probably cope with for the moment, though I may be asking questions.
If you have any beginner level tutorial, then that’s even better..I can just about cope with associated types, type families and GADT...some of the other extensions I suspect disbelief for.
> What does “~” mean…(it’s something that comes out in error messages
> when my types are all messed up)….
It means GHC knows that two types are equal.
> and then there’s a “=>” going on…in the middle of a signature….I know
> “=>” in the context of “Num a => a -> a -> a”
Yeah, that looks scary. You use gcastWith when you need a value of type a but you have a value of type b with a proof that a and b are equal (here a proof is a value of a :~: b). I have a practical example, though perhaps it's not the simplest one:
This code is thought as an intermediate level tutorial - you might want to give it a try. Another example I have is here (I use the name subst, but it's identical to gcastWith):
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