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

[Rustom Mody]

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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