[Haskell-cafe] Equality test between types that returns type-level Bool ?
muranushi at gmail.com
Tue Nov 27 00:42:16 CET 2012
Dear Gábor, Erik, and Oleg,
Thank you for your advices. Also what I have wanted, the extensible
dimensional type system, has just been released.
Now I have homeworks to test these, thank you!
2012/11/27 Erik Hesselink <hesselink at gmail.com>
> If you're up for it, Oleg has a lot of interesting material about this
> subject .
>  http://okmij.org/ftp/Haskell/typeEQ.html
> On Sun, Nov 25, 2012 at 9:36 AM, Takayuki Muranushi <muranushi at gmail.com>wrote:
>> 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.
>> Some of my attempts:
>> This fails because :==: is not an equality test between a and b, but
>> is a equality test within a (promoted) kind.
>> This fails because type instance declarations are not read from top to
>> bottom. (not like function declarations.)
>> 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.)
>> * We cannot write SameType, but there are ways to write functions like
>> 'filter' and 'merge' , over type-level lists, without using SameType.
>> Always grateful to your help,
>> Takayuki MURANUSHI
>> The Hakubi Center for Advanced Research, Kyoto University
>> Haskell-Cafe mailing list
>> Haskell-Cafe at haskell.org
The Hakubi Center for Advanced Research, Kyoto University
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