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

On Thu, Dec 27, 2012 at 11:48 PM, Rustom Mody wrote:

On Thu, Dec 27, 2012 at 8:26 PM, Kim-Ee Yeoh 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 wrote:

On Thu, Dec 27, 2012 at 8:26 PM, Kim-Ee Yeoh wrote:

...

Hi David, it looks like Rustom's aware that haskell's not lisp. What he

...

-- 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

really wants methinks is a way to suppress type classes altogether! That or a NoOverloadedNumerals extension. 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!