On Thu, Jun 13, 2019 at 11:15:23AM +0530, Sandeep.C.R via Haskell-Cafe wrote:

This seems to work for me...

someCheck :: forall a. (Show a, Read a, Eq a) => String -> a -> Bool someCheck s v = (read . show . (read :: String -> a) $ s) == v

Probably requires 'ScopedTypeVariables'...

Yes. Another variant is: {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeApplications #-} someCheck :: (Show a, Read a, Eq a) => String -> a -> Bool someCheck s (v :: a) = (read . show $ read @a s) == v which amounts to the same thing, but is perhaps simpler. For fun, with GHC 8.6 and "BlockArguments" we can drop some parentheses: {-# LANGUAGE BlockArguments #-} {-# LANGUAGE TypeApplications #-} {-# LANGUAGE ScopedTypeVariables #-} someCheck :: forall a. (Show a, Read a, Eq a) => String -> a -> Bool someCheck s v = v == do read $ show $ read @a s -- Viktor.

On Jun 13, 2019, at 4:25 AM, Boris wrote:

Agree, a good point. And actually, your variant works without any extensions.

Well, in a sense it is working with an "extension", just one that happens to be on by default and is required in Haskell 98. Namely, what makes it work is the "MonomorphismRestriction". If you specify: {-# LANGUAGE NoMonomorphismRestriction #-} then the example stops working. I like the posted example as an exceptionally clear illustration of the MonomorphismRestriction. -- Viktor.

Tom, Haha, nice. Now we have someCheck defined for so many scenarios. I like it. Didn't expect to get so many answers ;) Thanks everyone! Cheers, boris@d12frosted.io ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ On Thursday, June 13, 2019 12:53 PM, Tom Ellis wrote:

On Thu, Jun 13, 2019 at 04:46:25AM -0400, Viktor Dukhovni wrote:

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On Jun 13, 2019, at 4:25 AM, Boris boris@d12frosted.io wrote: Agree, a good point. And actually, your variant works without any extensions.

Well, in a sense it is working with an "extension", just one that happens to be on by default and is required in Haskell 98. Namely, what makes it work is the "MonomorphismRestriction". If you specify: {-# LANGUAGE NoMonomorphismRestriction #-} then the example stops working. I like the posted example as an exceptionally clear illustration of the MonomorphismRestriction.

And for the record, this version requires no extension nor unextension:

{-# LANGUAGE NoMonomorphismRestriction #-}

module Check where

someCheck :: (Show a, Read a, Eq a) => String -> a -> Bool

someCheck = someCheck' read where someCheck' r s v = (r . show . r $ s) == v

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