[Haskell-cafe] Equality test between types that returns type-level Bool ?

[Takayuki Muranushi]

By tracing how unittyped produced the 'True-s and 'False-s in the error
messages, and by Oleg's lecture,

> 1 meter + 5 second

<interactive>:17:9:
    Couldn't match type 'False with 'True
    When using functional dependencies to combine
      UnitTyped.And 'False 'False 'False,
        arising from the dependency `a b -> c'
        in the instance declaration in `UnitTyped'
      UnitTyped.And 'False 'False 'True,
        arising from a use of `+' at <interactive>:17:9
    In the expression: 1 meter + 5 second
    In an equation for `it': it = 1 meter + 5 second


I understood how type-level equalities
https://github.com/nushio3/dimensional-tf/blob/master/attic/typeeq-01.hs
and type-level list lookups
https://github.com/nushio3/dimensional-tf/blob/master/attic/typeeq-03.hs

can be implemented using overlapped instances. Thank you for the
instructions.

and I'm looking forward to see TYPEREP with ghc7.6.1's promoted integers
and TH pretty soon!


2012/11/27 Takayuki Muranushi <muranushi at gmail.com>

> Dear Gábor, Erik, and Oleg,
>
> Thank you for your advices. Also what I have wanted, the extensible
> dimensional type system, has just been released.
>
> http://hackage.haskell.org/package/unittyped-0.1
>
> 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 [1].
>>
>> Regards,
>>
>> Erik
>>
>> [1] 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.
>>>
>>>
>>> http://hackage.haskell.org/packages/archive/dimensional/0.10.2/doc/html/Numeric-Units-Dimensional-Extensible.html
>>>
>>> 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.)
>>> * 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
>>> http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html
>>>
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe at haskell.org
>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>>
>>
>>
>
>
> --
> Takayuki MURANUSHI
> The Hakubi Center for Advanced Research, Kyoto University
> http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html
>



-- 
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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