Prelude> :t [[1,2],3] you have a list with 2 elements: - [1,2] - 3 the type of [1,2] is [Integer] the type of 3 is Integer But all elements in a list must have the same type. 2012/12/27 Rustom Mody :

On Thu, Dec 27, 2012 at 1:48 AM, Roman Cheplyaka wrote:

...

* Rustom Mody [2012-12-26 20:12:17+0530]

...

So is there any set of flags to make haskell literals less polymorphic?

Yes, there is!

% ghci -XRebindableSyntax GHCi, version 7.6.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done.

...

import Prelude hiding (fromInteger) Prelude> let fromInteger = id Prelude> :t 3 3 :: Integer

Roman

Thanks Roman -- that helps. And yet the ghci error is much more obscure than the gofer error:

--- contents of .ghci --- :set -XRebindableSyntax let fromInteger = id ------ ghci session ----- $ 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 5 5 :: Integer 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]

----- The same in gofer ----- Gofer session for: pustd.pre ? :t [[1,2],3]

ERROR: Type error in list *** expression : [[1,2],3]

*** term : 3 *** type : Int *** does not match : [Int] -------------- So the error is occurring at the point of the fromInteger (= id) but the message does not indicate that

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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 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!

* Rustom Mody [2012-12-27 22:18:15+0530]

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> (/)

It's because RebindableSyntax implies NoImplicitPrelude. This is not an issue if you only work in the interpreter (you can put "import Prelude hiding (fromInteger)" in .ghci), but you'd also need to put that into every source file that you wish to load. An alternative would be to create your own Prelude (or use an existing one, like [1]) and use it instead of the one defined in base (by hiding base and exposing a different package). [1]: http://hackage.haskell.org/packages/archive/simpleprelude/1.0.1.3/doc/html/P... Roman

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!