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@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@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@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

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--
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