[Haskell-cafe] Non polymorphic numerals option -- avoiding type classes

[Kim-Ee Yeoh]

On Thu, Dec 27, 2012 at 11:48 PM, Rustom Mody <rustompmody at gmail.com> wrote:
> On Thu, Dec 27, 2012 at 8:26 PM, Kim-Ee Yeoh <ky3 at atamo.com> wrote:
>> What he really wants methinks is a way to suppress type classes
altogether! That or a NoOverloadedNumerals extension.
>
> I'm not really sure about that... Look!
>
> Prelude> :t [[1,2],3]
> [[1,2],3] :: (Num [t], Num t) => [[t]]

As Satvik explained, well-typed does not imply instantiable. And with
constraints, not instantiable /does/ imply not evaluable!

> :set -XRebindableSyntax
> let fromInteger = id
> Prelude> :t [[1,2],3]
>
>     Couldn't match expected type `[Integer]' with actual type `Integer'
>     Expected type: Integer -> [Integer]
>       Actual type: Integer -> Integer
>     In the expression: 3
>     In the expression: [[1, 2], 3]

You can see overloaded numerals at work again via the hidden hand of
fromInteger.

Presumably some imaginary NoOverloadedNumerals extension would thoroughly
purge its presence.

-- Kim-Ee


On Thu, Dec 27, 2012 at 11:48 PM, Rustom Mody <rustompmody at gmail.com> wrote:
>
> On Thu, Dec 27, 2012 at 8:26 PM, Kim-Ee Yeoh <ky3 at atamo.com> wrote:
>>
>> Hi David, it looks like Rustom's aware that haskell's not lisp. What he
really wants methinks is a way to suppress type classes altogether! That or
a NoOverloadedNumerals extension.
>>
>> -- Kim-Ee
>>
>
> I'm not really sure about that... Look!
>
> ghci with default startup
> --------------------
>
> $ ghci
> GHCi, version 7.4.1: http://www.haskell.org/ghc/  :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer-gmp ... linking ... done.
> Loading package base ... linking ... done.
> Prelude> :t [[1,2],3]
> [[1,2],3] :: (Num [t], Num t) => [[t]]
>
> So it would appear that ghci is giving a well-typing for [[1,2], 3].
> But is it?
>
> Prelude> [[1,2],3]
>
> <interactive>:3:8:
>
>     No instance for (Num [t0])
>       arising from the literal `3'
>
>     Possible fix: add an instance declaration for (Num [t0])
>     In the expression: 3
>     In the expression: [[1, 2], 3]
>     In an equation for `it': it = [[1, 2], 3]
> -------------------
> So is it well-typed in ghci or not??
>
> And  now we add Roman's suggestions...
> $ cat .ghci
>
> :set -XRebindableSyntax
> let fromInteger = id
>
> And run ghci again
>
>
> $ ghci
> GHCi, version 7.4.1: http://www.haskell.org/ghc/  :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer-gmp ... linking ... done.
> Loading package base ... linking ... done.
> Prelude> :t [[1,2],3]
>
> <interactive>:1:8:
>     Couldn't match expected type `[Integer]' with actual type `Integer'
>     Expected type: Integer -> [Integer]
>       Actual type: Integer -> Integer
>     In the expression: 3
>     In the expression: [[1, 2], 3]
> Prelude> [[1,2],3]
>
> <interactive>:3:8:
>
>     Couldn't match expected type `[Integer]' with actual type `Integer'
>     Expected type: Integer -> [Integer]
>       Actual type: Integer -> Integer
>     In the expression: 3
>     In the expression: [[1, 2], 3]
> Prelude>
>
> So far so good -- when an expression is type-wrong, its 'wrongness' is
the same irrespective of whether I ask for its type or evaluate it.
>
> But now we are in for new surprises:  Try out
> f x y = x / y
> Prelude> :l f
> [1 of 1] Compiling Main             ( f.hs, interpreted )
>
> f.hs:1:11: Not in scope: `/'
> Failed, modules loaded: none.
> Prelude> (/)
>
> Oh is it that now integer literals are just plain Integers and cant be
divided using '/' ??
>
> So lets replace '/' with '+'
> f.hs:1:11: Not in scope: `+'
>
> And now I am at my wits end!
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