RE: [Haskell-cafe] function types as instances of Num
Try
test' = square . (4 :: a -> (Integer,a))
Otherwise, how is the compiler to know that you want 4 to be of that
type?
S
| -Original Message-
| From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Greg
| Buchholz
| Sent: 26 October 2006 18:46
| To: haskell-cafe@haskell.org
| Subject: [Haskell-cafe] function types as instances of Num
|
|
| Let's say we've got a little stack language, where you compute
| things by transformations of stacks, using compositions of functions
| from stacks to stacks (represented here as nested tuples). (See also
| Chris Okasaki's "Techniques for Embedding Postfix Languages in
Haskell"
| www.eecs.harvard.edu/~nr/ cs252r/archive/chris-okasaki/hw02.ps )
|
| For example, the simple program below calculates the square of 4...
|
| > {-# OPTIONS -fglasgow-exts #-}
| >
| > main = print $ test ()
| >
| > test = square . (lit 4)
| >
| > lit :: Integer -> a -> (Integer,a)
| > lit val stack= (val, stack)
| >
| > dup (a, b) = (a, (a, b))
| > mult (a, (b, c)) = (b*a, c)
| > square = mult . dup
|
| ...now let's say I find that using the function "lit" to annotation
| numeric literals ugly. What I really want is something like...
|
| > test' = square . 4
|
| ...Seems simple enough, I'll just make an appropriate instance of Num
| and I'll be able to use fromInteger...
|
| > instance Eq (a -> (Integer, a))
| > instance Show (a -> (Integer, a))
| > instance Num (a -> (Integer, a)) where
| > fromInteger = lit
|
| ...but now when I try it, GHC complains...
|
| No instance for (Num (a -> (Integer, t)))
| arising from the literal `4' at final.hs:15:17
| Possible fix:
| add an instance declaration for (Num (a -> (Integer, t)))
| In the second argument of `(.)', namely `4'
| In the expression: square . 4
| In the definition of `test'': test' = square . 4
|
| ...so it seems that (a -> (Integer, t)) can't be unified with (a ->
| (Integer, a)), or somesuch. Any thoughts on how to get this to work?
|
|
| Thanks,
|
| Greg Buchholz
|
|
| ___
| Haskell-Cafe mailing list
| Haskell-Cafe@haskell.org
| http://www.haskell.org/mailman/listinfo/haskell-cafe
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Re: [Haskell-cafe] function types as instances of Num
You need to monomorphize the result before printing: main = print $ ((square . 4) :: Alpha ()) Presumably you will apply (square . 4) at some point to a concrete state at some point, and you wouldn't need to provide the type explicitly. Greg Buchholz wrote: Dan Weston wrote: How about: Hmm. I'm probably being dense today, but when I add the following definitions to your program... main = print $ (square . 4) () square (a,b) = (a*a,b) ...I still get the same error... No instance for (Num (() -> (t, t1))) arising from the literal `4' at weston.hs:5:25 Possible fix: add an instance declaration for (Num (() -> (t, t1))) In the second argument of `(.)', namely `4' In the second argument of `($)', namely `(square . 4) ()' In the expression: print $ ((square . 4) ()) ...maybe you could show me your implementation of "main" and "square" to help nudge me in the right direction. (I'm using ghc-6.6) Thanks, Greg Buchholz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] function types as instances of Num
Dan Weston wrote: > How about: Hmm. I'm probably being dense today, but when I add the following definitions to your program... main = print $ (square . 4) () square (a,b) = (a*a,b) ...I still get the same error... No instance for (Num (() -> (t, t1))) arising from the literal `4' at weston.hs:5:25 Possible fix: add an instance declaration for (Num (() -> (t, t1))) In the second argument of `(.)', namely `4' In the second argument of `($)', namely `(square . 4) ()' In the expression: print $ ((square . 4) ()) ...maybe you could show me your implementation of "main" and "square" to help nudge me in the right direction. (I'm using ghc-6.6) Thanks, Greg Buchholz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] function types as instances of Num
How about:
{-# OPTIONS -fglasgow-exts #-}
import Control.Arrow
type Alpha alpha = alpha -> (Integer,alpha)
test = square . (lit 4)
lit :: Integer -> Alpha alpha
lit val stack= (val, stack)
instance Eq (Alpha alpha) where
x == y = uncurry (==) . (fst . x &&& fst . y) $ undefined
instance Show (Alpha alpha) where
show x = show . fst $ x undefined
instance Num (Alpha alpha) where
fromInteger i = (\s -> (i,s))
(+) = fBinary (+)
(-) = fBinary (-)
(*) = fBinary (*)
negate= fUnary negate
abs = fUnary abs
signum= fUnary signum
fUnary op x = (op . fst &&& snd ) . x
fBinary op x y = (uncurry op . (fst *** fst) &&& (snd . fst)) . (x &&& y)
Greg Buchholz wrote:
> Let's say we've got a little stack language, where you compute
> things by transformations of stacks, using compositions of functions
> from stacks to stacks (represented here as nested tuples). (See also
> Chris Okasaki's "Techniques for Embedding Postfix Languages in Haskell"
> www.eecs.harvard.edu/~nr/ cs252r/archive/chris-okasaki/hw02.ps )
>
> For example, the simple program below calculates the square of 4...
>> {-# OPTIONS -fglasgow-exts #-}
>>
>> main = print $ test ()
>> test = square . (lit 4)
>>
>> lit :: Integer -> a -> (Integer,a)
>> lit val stack= (val, stack)
>>
>> dup (a, b) = (a, (a, b))
>> mult (a, (b, c)) = (b*a, c)
>> square = mult . dup
>
> ...now let's say I find that using the function "lit" to annotation
> numeric literals ugly. What I really want is something like...
>
>> test' = square . 4
>
> ...Seems simple enough, I'll just make an appropriate instance of Num
> and I'll be able to use fromInteger...
>
>> instance Eq (a -> (Integer, a)) instance Show (a -> (Integer, a))
instance Num (a -> (Integer, a)) where
>> fromInteger = lit
>
> ...but now when I try it, GHC complains...
>
> No instance for (Num (a -> (Integer, t)))
> arising from the literal `4' at final.hs:15:17
> Possible fix:
> add an instance declaration for (Num (a -> (Integer, t)))
> In the second argument of `(.)', namely `4'
> In the expression: square . 4
> In the definition of `test'': test' = square . 4
>
> ...so it seems that (a -> (Integer, t)) can't be unified with (a ->
> (Integer, a)), or somesuch. Any thoughts on how to get this to work?
>
>
> Thanks,
>
> Greg Buchholz
>
>
> ___
> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>
___
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[Haskell-cafe] function types as instances of Num
Let's say we've got a little stack language, where you compute
things by transformations of stacks, using compositions of functions
from stacks to stacks (represented here as nested tuples). (See also
Chris Okasaki's "Techniques for Embedding Postfix Languages in Haskell"
www.eecs.harvard.edu/~nr/ cs252r/archive/chris-okasaki/hw02.ps )
For example, the simple program below calculates the square of 4...
> {-# OPTIONS -fglasgow-exts #-}
>
> main = print $ test ()
>
> test = square . (lit 4)
>
> lit :: Integer -> a -> (Integer,a)
> lit val stack= (val, stack)
>
> dup (a, b) = (a, (a, b))
> mult (a, (b, c)) = (b*a, c)
> square = mult . dup
...now let's say I find that using the function "lit" to annotation
numeric literals ugly. What I really want is something like...
> test' = square . 4
...Seems simple enough, I'll just make an appropriate instance of Num
and I'll be able to use fromInteger...
> instance Eq (a -> (Integer, a))
> instance Show (a -> (Integer, a))
> instance Num (a -> (Integer, a)) where
> fromInteger = lit
...but now when I try it, GHC complains...
No instance for (Num (a -> (Integer, t)))
arising from the literal `4' at final.hs:15:17
Possible fix:
add an instance declaration for (Num (a -> (Integer, t)))
In the second argument of `(.)', namely `4'
In the expression: square . 4
In the definition of `test'': test' = square . 4
...so it seems that (a -> (Integer, t)) can't be unified with (a ->
(Integer, a)), or somesuch. Any thoughts on how to get this to work?
Thanks,
Greg Buchholz
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