#3005: Normalize fully-applied type functions prior to display
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Reporter: dmcclean | Owner:
Type: feature request | Status: new
Priority: normal | Component: GHCi
Version: 6.10.1 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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I have a module with a matrix type constructor:
> data (Natural r, Natural c) => Matrix r c t = Matrix [[t]] deriving (Eq)
which uses type-level naturals to provide dimension checking.
A type class for multiplication:
> class Multiply a b where
> type Product a b
> times :: a -> b -> Product a b
and an instance for matrix multiplication:
> instance (Natural a, Natural b, Natural c, Num t) => Multiply (Matrix a
b t) (Matrix b c t) where
> type Product (Matrix a b t) (Matrix b c t) = Matrix a c t
> times m1 m2 = ...
All of this works really well, I get dimension checking (and inference),
and lot of other goodies.
My request has to do with the following GHCi session:
*Main> let a = matrix two two [[1,1],[2,6]]
*Main> :t a
a :: Matrix Two Two Integer
*Main> :t a `times` a
a `times` a :: Product
(Matrix Two Two Integer) (Matrix Two Two Integer)
When a type function is fully-applied, as Product is in this case, it
would be nice if GHC could simplify it prior to display.
This would result in the following output for this example:
:t a `times` a
a `times` a :: Matrix Two Two Integer
I have attached the files necessary to run this test case, although a
simpler one could be constructed.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3005>
GHC <http://www.haskell.org/ghc/>
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