Re: [Haskell-cafe] a code that cannot compile with or without NoMonomorphismRestriction
f :: (Show a, Ord b) = (a - String, b - b - Bool)
f = let commond_definitions = undefined in
let f1 = id.show
f2 x = ( x)
in
(f1, f2)
From: Ting Lei tin...@hotmail.com
To: haskell-cafe@haskell.org
Cc:
Date: Wed, 28 Mar 2012 23:42:30 -0700
Subject: [Haskell-cafe] a code that cannot compile with or without
NoMonomorphismRestriction
Hi
I have met a piece of code that cannot be compiled whether I add or remove
the NoMonomorphismRestriction flag (as of GHC 7.0.4, Haskell platform
2011.4.0.0).
I have extracted a minimal example below:
{-# LANGUAGE NoMonomorphismRestriction #-}
(f1, f2) =
let commond_definitions = undefined in
let f1 = id.show
f2 x = ( x)
in
(f1, f2)
I needed this format because there are many shared definitions in
common_definitions for f1 and f2, and I want to keep them local.
If I compile them with NoMonomorphismRestriction, I get:
D:\work\test.hs:7:8:
Ambiguous type variable `a0' in the constraint:
(Show a0) arising from a use of `f1'
Possible cause: the monomorphism restriction applied to the following:
f1 :: a0 - String (bound at D:\work\hsOcaml\test.hs:2:2)
Probable fix: give these definition(s) an explicit type signature
In the expression: f1
In the expression: (f1, f2)
In the expression:
let
f1 = id . show
f2 x = ( x)
in (f1, f2)
D:\work\test.hs:7:12:
Ambiguous type variable `a1' in the constraint:
(Ord a1) arising from a use of `f2'
Possible cause: the monomorphism restriction applied to the following:
f2 :: a1 - a1 - Bool (bound at D:\work\hsOcaml\test.hs:2:6)
Probable fix: give these definition(s) an explicit type signature
In the expression: f2
In the expression: (f1, f2)
In the expression:
let
f1 = id . show
f2 x = ( x)
in (f1, f2)
Failed, modules loaded: none.
If I comment out
-- {-# LANGUAGE NoMonomorphismRestriction #-}
I get:
D:\work\hsOcaml\test.hs:4:17:
Ambiguous type variable `a0' in the constraint:
(Show a0) arising from a use of `show'
Possible cause: the monomorphism restriction applied to the following:
f1 :: a0 - String (bound at D:\work\hsOcaml\test.hs:2:2)
Probable fix: give these definition(s) an explicit type signature
or use -XNoMonomorphismRestriction
In the second argument of `(.)', namely `show'
In the expression: id . show
In an equation for `f1': f1 = id . show
D:\work\hsOcaml\test.hs:7:12:
Ambiguous type variable `a1' in the constraint:
(Ord a1) arising from a use of `f2'
Possible cause: the monomorphism restriction applied to the following:
f2 :: a1 - a1 - Bool (bound at D:\work\hsOcaml\test.hs:2:6)
Probable fix: give these definition(s) an explicit type signature
or use -XNoMonomorphismRestriction
In the expression: f2
In the expression: (f1, f2)
In the expression:
let
f1 = id . show
f2 x = ( x)
in (f1, f2)
Failed, modules loaded: none.
Can anyone show me why this does not work and how to fix it (e.g. by
adding type signature as the error message suggested)?
I tried to add type signature by couldn't figure out the right way of
doing it.
Thanks in advance!
Ting
Denis Moskvin
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Re: [Haskell-cafe] a code that cannot compile with or without NoMonomorphismRestriction
Ting Lei tin...@hotmail.com writes: (f1, f2) = let commond_definitions = undefined in let f1 = id.show f2 x = ( x) in (f1, f2) I think the type signatures should be: f1 :: Show a = a - String and f2 :: Ord b = b - b - Bool When I define these separately, this works: f1 :: Show a = a - String f1 = id . show f2 :: Ord b = b - b - Bool f2 = flip () But when I define them as a pair f1 :: Show a = a - String f2 :: Ord b = b - b - Bool (f1,f2) = (id . show, flip ()) I get an error message: Line 9: 1 error(s), 0 warning(s) Couldn't match expected type `forall a. Show a = a - String' with actual type `a - String' When checking that `f1' has the specified type `forall a1. Show a1 = a1 - String' Defining the pair at once works: p :: (Show a, Ord b) = (a - String, b - b - Bool) p = (id . show, flip ()) I guess that didn't help a lot, somebody with deeper GHC-fu than me will have to step in. -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] a code that cannot compile with or without NoMonomorphismRestriction
Ketil Malde wrote: Ting Lei tin...@hotmail.com writes: (f1, f2) = let commond_definitions = undefined in let f1 = id.show f2 x = ( x) in (f1, f2) I think the type signatures should be: f1 :: Show a = a - String and f2 :: Ord b = b - b - Bool When I define these separately, this works: f1 :: Show a = a - String f1 = id . show f2 :: Ord b = b - b - Bool f2 = flip () But when I define them as a pair f1 :: Show a = a - String f2 :: Ord b = b - b - Bool (f1,f2) = (id . show, flip ()) I get an error message: Line 9: 1 error(s), 0 warning(s) Couldn't match expected type `forall a. Show a = a - String' with actual type `a - String' When checking that `f1' has the specified type `forall a1. Show a1 = a1 - String' Defining the pair at once works: p :: (Show a, Ord b) = (a - String, b - b - Bool) p = (id . show, flip ()) I guess that didn't help a lot, somebody with deeper GHC-fu than me will have to step in. The problem is that f1 and f2 are polymorphic functions. To put polymorphic functions in a pair, you need *impredicative polymorphism*. Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] a code that cannot compile with or without NoMonomorphismRestriction
I think the error message tell you how to fix:
use -XNoMonomorphismRestriction
One approach is add following line into top of your hs file and it works for me.
{-# LANGUAGE NoMonomorphismRestriction #-}
Regarding the deeper reason, I think you would be able to find via GHC
user guide and google.
-Haisheng
On Thu, Mar 29, 2012 at 2:42 PM, Ting Lei tin...@hotmail.com wrote:
Hi
I have met a piece of code that cannot be compiled whether I add or remove
the NoMonomorphismRestriction flag (as of GHC 7.0.4, Haskell platform
2011.4.0.0).
I have extracted a minimal example below:
{-# LANGUAGE NoMonomorphismRestriction #-}
(f1, f2) =
let commond_definitions = undefined in
let f1 = id.show
f2 x = ( x)
in
(f1, f2)
I needed this format because there are many shared definitions in
common_definitions for f1 and f2, and I want to keep them local.
If I compile them with NoMonomorphismRestriction, I get:
D:\work\test.hs:7:8:
Ambiguous type variable `a0' in the constraint:
(Show a0) arising from a use of `f1'
Possible cause: the monomorphism restriction applied to the following:
f1 :: a0 - String (bound at D:\work\hsOcaml\test.hs:2:2)
Probable fix: give these definition(s) an explicit type signature
In the expression: f1
In the expression: (f1, f2)
In the expression:
let
f1 = id . show
f2 x = ( x)
in (f1, f2)
D:\work\test.hs:7:12:
Ambiguous type variable `a1' in the constraint:
(Ord a1) arising from a use of `f2'
Possible cause: the monomorphism restriction applied to the following:
f2 :: a1 - a1 - Bool (bound at D:\work\hsOcaml\test.hs:2:6)
Probable fix: give these definition(s) an explicit type signature
In the expression: f2
In the expression: (f1, f2)
In the expression:
let
f1 = id . show
f2 x = ( x)
in (f1, f2)
Failed, modules loaded: none.
If I comment out
-- {-# LANGUAGE NoMonomorphismRestriction #-}
I get:
D:\work\hsOcaml\test.hs:4:17:
Ambiguous type variable `a0' in the constraint:
(Show a0) arising from a use of `show'
Possible cause: the monomorphism restriction applied to the following:
f1 :: a0 - String (bound at D:\work\hsOcaml\test.hs:2:2)
Probable fix: give these definition(s) an explicit type signature
or use -XNoMonomorphismRestriction
In the second argument of `(.)', namely `show'
In the expression: id . show
In an equation for `f1': f1 = id . show
D:\work\hsOcaml\test.hs:7:12:
Ambiguous type variable `a1' in the constraint:
(Ord a1) arising from a use of `f2'
Possible cause: the monomorphism restriction applied to the following:
f2 :: a1 - a1 - Bool (bound at D:\work\hsOcaml\test.hs:2:6)
Probable fix: give these definition(s) an explicit type signature
or use -XNoMonomorphismRestriction
In the expression: f2
In the expression: (f1, f2)
In the expression:
let
f1 = id . show
f2 x = ( x)
in (f1, f2)
Failed, modules loaded: none.
Can anyone show me why this does not work and how to fix it (e.g. by adding
type signature as the error message suggested)?
I tried to add type signature by couldn't figure out the right way of doing
it.
Thanks in advance!
Ting
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Re: [Haskell-cafe] a code that cannot compile with or without NoMonomorphismRestriction
Ketil, Thanks for the response. It seems that defining them as a pair only
postphones the error.GHC will give an error when you extract the components of
the pair, no matter whether you addthe NoMonomorphismRestriction flag or not.
--{-# LANGUAGE NoMonomorphismRestriction #-}p :: (Show a, Ord b) = (a -
String, b - b - Bool)
p = (id . show, flip ())f1 = fst p
f2 = snd p---Without NoMonomorphismRestriction, I got:
D:\work\test1.hs:6:10:
Ambiguous type variable `a0' in the constraint:
(Show a0) arising from a use of `p'
Possible cause: the monomorphism restriction applied to the following:
f1 :: a0 - String (bound at D:\work\hsOcaml\test1.hs:6:1)
Probable fix: give these definition(s) an explicit type signature
or use -XNoMonomorphismRestriction
In the first argument of `fst', namely `p'
In the expression: fst p
In an equation for `f1': f1 = fst pD:\work\hsOcaml\test1.hs:6:10:
Ambiguous type variable `b0' in the constraint:
(Ord b0) arising from a use of `p'
Probable fix: add a type signature that fixes these type variable(s)
.. Failed, modules loaded: none. --With
NoMonomorphismRestriction, I got:
D:\work\test1.hs:6:10:
Ambiguous type variable `b0' in the constraint:
(Ord b0) arising from a use of `p'
Probable fix: add a type signature that fixes these type variable(s)
In the first argument of `fst', namely `p'
In the expression: fst p
In an equation for `f1': f1 = fst pD:\work\test1.hs:7:10:
Ambiguous type variable `a0' in the constraint:
(Show a0) arising from a use of `p'
Probable fix: add a type signature that fixes these type variable(s)
In the first argument of `snd', namely `p'
In the expression: snd p
In an equation for `f2': f2 = snd p
Failed, modules loaded: none. Thanks,
Ting From: ke...@malde.org
To: tin...@hotmail.com
CC: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] a code that cannot compile with or without
NoMonomorphismRestriction
Date: Thu, 29 Mar 2012 12:27:04 +0200
Ting Lei tin...@hotmail.com writes:
(f1, f2) =
let commond_definitions = undefined in
let f1 = id.show
f2 x = ( x)
in
(f1, f2)
I think the type signatures should be:
f1 :: Show a = a - String
and
f2 :: Ord b = b - b - Bool
When I define these separately, this works:
f1 :: Show a = a - String
f1 = id . show
f2 :: Ord b = b - b - Bool
f2 = flip ()
But when I define them as a pair
f1 :: Show a = a - String
f2 :: Ord b = b - b - Bool
(f1,f2) = (id . show, flip ())
I get an error message:
Line 9: 1 error(s), 0 warning(s)
Couldn't match expected type `forall a. Show a = a - String'
with actual type `a - String'
When checking that `f1'
has the specified type `forall a1. Show a1 = a1 - String'
Defining the pair at once works:
p :: (Show a, Ord b) = (a - String, b - b - Bool)
p = (id . show, flip ())
I guess that didn't help a lot, somebody with deeper GHC-fu than me will
have to step in.
-k
--
If I haven't seen further, it is by standing in the footprints of giants
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