On Sun, Nov 25, 2012 at 9:36 AM, Takayuki Muranushi
Is it possible to write
type family SameType a b :: Bool
which returns True if a and b are the same type, and False otherwise?
I encountered this problem when I was practicing promoted lists and tuples in ghc-7.6.1. One of my goal for practice is to write more "modular" version of extensible-dimensional calculations, and to understand whether ghc-7.6.1 is capable of it.
http://hackage.haskell.org/packages/archive/dimensional/0.10.2/doc/html/Nume...
Some of my attempts:
https://github.com/nushio3/dimensional-tf/blob/master/attic/list-02.hs This fails because :==: is not an equality test between a and b, but is a equality test within a (promoted) kind.
https://github.com/nushio3/dimensional-tf/blob/master/attic/list-03.hs This fails because type instance declarations are not read from top to bottom. (not like function declarations.)
https://github.com/nushio3/dimensional-tf/blob/master/attic/map-03.hs I could define a lookup using class constraints, but when I use it, results in overlapping instances.
So, will somebody teach me which of the following is correct?
* We can write a type family SameType a b :: Bool * We cannot do that because of theoretical reason (that leads to non-termination etc.)
This is correct. It's not currently possible to have overlapping type family instances without breaking the soundness of the type system. There's some work on changing it: http://hackage.haskell.org/trac/ghc/wiki/NewAxioms
* We cannot write SameType, but there are ways to write functions like 'filter' and 'merge' , over type-level lists, without using SameType.
Not easily that I know of. There's no way to write a general "else", "otherwise", "if not", etc. case. You have to explicitly enumerate every type you want to use it with. It _is_ possible with type classes and OverlappingInstances, but beyond the trivial cases it tends to make my head hurt (and there's no way to get a result "out" of a type class "computation" and into a type family).
Always grateful to your help, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html
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