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Today's Topics:
1. Is this a GHC bug? Problem inferring type (Peter Hall)
2. Re: Is this a GHC bug? Problem inferring type (Brandon Allbery)
3. Re: Is this a GHC bug? Problem inferring type (Peter Hall)
4. Re: Is this a GHC bug? Problem inferring type (Michael Peternell)
5. Re: Is this a GHC bug? Problem inferring type (Brandon Allbery)
6. Re: Is this a GHC bug? Problem inferring type (Peter Hall)
7. Re: Is this a GHC bug? Problem inferring type (Graham Gill)
----------------------------------------------------------------------
Message: 1
Date: Sat, 8 Jun 2013 14:02:02 +0100
From: Peter Hall <[email protected]>
Subject: [Haskell-beginners] Is this a GHC bug? Problem inferring type
To: "[email protected]" <[email protected]>
Message-ID:
<CAA6hAk4LupBkJh=0UyFTPqx=hw--bnri2kcxmyjthjdul+u...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
This is confusing me, and I'm wondering if it's a compiler bug. I have this:
foo = (++) `on` show
I'm expecting the type to be:
foo :: Show a => a -> a -> String
But GHCI is inferring:
foo :: () -> () -> String
If I explicitly provide a type annotation, or if I use
-XNoMonormorphismRestriction, then it all works fine. I would expect a
compile error if the type couldn't be inferred due to the Monomorphism
Restriction, but instead it has silently chosen the least useful type.
Peter
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Message: 2
Date: Sat, 8 Jun 2013 09:35:52 -0400
From: Brandon Allbery <[email protected]>
Subject: Re: [Haskell-beginners] Is this a GHC bug? Problem inferring
type
To: Peter Hall <[email protected]>, The Haskell-Beginners
Mailing List - Discussion of primarily beginner-level topics related
to Haskell <[email protected]>
Message-ID:
<cakfcl4ue_0ozbycjafr1qexhz_e23qs7mswoxjw3uertprs...@mail.gmail.com>
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On Sat, Jun 8, 2013 at 9:02 AM, Peter Hall <[email protected]> wrote:
> This is confusing me, and I'm wondering if it's a compiler bug. I have
> this:
>
> foo = (++) `on` show
>
> I'm expecting the type to be:
>
> foo :: Show a => a -> a -> String
>
> But GHCI is inferring:
>
> foo :: () -> () -> String
>
No bug; it's the monomorphism restriction combining with extended
defaulting to infer a monomorphic type. If you want to keep the
monomorphism restriction and throw a type error, disable
ExtendedDefaultRules.
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 3
Date: Sat, 8 Jun 2013 14:39:31 +0100
From: Peter Hall <[email protected]>
Subject: Re: [Haskell-beginners] Is this a GHC bug? Problem inferring
type
To: Brandon Allbery <[email protected]>
Cc: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Message-ID:
<caa6hak5delnnfn5teldcjtss8r+rwfdaech-qqrgq8ofknq...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Ah, thanks for the explanation. I hadn't heard of ExtendedDefaultRules
before.
Peter
On 8 June 2013 14:35, Brandon Allbery <[email protected]> wrote:
> On Sat, Jun 8, 2013 at 9:02 AM, Peter Hall <[email protected]>wrote:
>
>> This is confusing me, and I'm wondering if it's a compiler bug. I have
>> this:
>>
>> foo = (++) `on` show
>>
>> I'm expecting the type to be:
>>
>> foo :: Show a => a -> a -> String
>>
>> But GHCI is inferring:
>>
>> foo :: () -> () -> String
>>
>
> No bug; it's the monomorphism restriction combining with extended
> defaulting to infer a monomorphic type. If you want to keep the
> monomorphism restriction and throw a type error, disable
> ExtendedDefaultRules.
>
> --
> brandon s allbery kf8nh sine nomine
> associates
> [email protected]
> [email protected]
> unix, openafs, kerberos, infrastructure, xmonad
> http://sinenomine.net
>
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Message: 4
Date: Sat, 8 Jun 2013 20:54:44 +0200
From: Michael Peternell <[email protected]>
Subject: Re: [Haskell-beginners] Is this a GHC bug? Problem inferring
type
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
in my opinion this is a bug of ghc. maybe the implementation works exactly as
it should, according to the specification: then it's a bug in the specification.
Show a => a -> a -> String
just makes much much more sense than
() -> () -> String
which makes no sense at all imho
and i think it would be possible to infer the type in this case.
++ :: [a] -> [a] -> [a]
show :: Show e => e -> String
on :: (b -> b -> c) -> (d -> b) -> d -> d -> c
on (++) show :: d -> d -> c
(++) :: [a] -> [a] -> [a] = b -> b -> c
-> b = c = [a]
show :: Show e => e -> String = (d -> b)
-> b = String, Show d
-> c = String, a = Char
on (++) show :: Show d => d -> d -> String
is it really not possible to write an algorithm that can deduce this kind of
stuff?
Am 08.06.2013 um 15:35 schrieb Brandon Allbery <[email protected]>:
> On Sat, Jun 8, 2013 at 9:02 AM, Peter Hall <[email protected]> wrote:
> This is confusing me, and I'm wondering if it's a compiler bug. I have this:
>
> foo = (++) `on` show
>
> I'm expecting the type to be:
>
> foo :: Show a => a -> a -> String
>
> But GHCI is inferring:
>
> foo :: () -> () -> String
>
> No bug; it's the monomorphism restriction combining with extended defaulting
> to infer a monomorphic type. If you want to keep the monomorphism restriction
> and throw a type error, disable ExtendedDefaultRules.
>
> --
> brandon s allbery kf8nh sine nomine associates
> [email protected] [email protected]
> unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
------------------------------
Message: 5
Date: Sat, 8 Jun 2013 15:33:50 -0400
From: Brandon Allbery <[email protected]>
Subject: Re: [Haskell-beginners] Is this a GHC bug? Problem inferring
type
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Message-ID:
<CAKFCL4XP55S4_z-T=jp_o27heznxacagvenmlncefdgn8qo...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Sat, Jun 8, 2013 at 2:54 PM, Michael Peternell
<[email protected]>wrote:
> in my opinion this is a bug of ghc. maybe the implementation works exactly
> as it should, according to the specification: then it's a bug in the
> specification.
>
> Show a => a -> a -> String
>
...is not monomorphic. If you want that to be inferred, turn off the
monomorphism restriction. (And yes, many people *do* consider the
monomorphism restriction of standard Haskell to be a bug in the
specification.)
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 6
Date: Sat, 8 Jun 2013 21:11:13 +0100
From: Peter Hall <[email protected]>
Subject: Re: [Haskell-beginners] Is this a GHC bug? Problem inferring
type
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Message-ID:
<CAA6hAk4Xo-b7-Z0iMZNuuBK96y7d6Xt8323wUc1ysJEOa=q...@mail.gmail.com>
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>
>> Show a => a -> a -> String
>>
>
> ...is not monomorphic. If you want that to be inferred, turn off the
> monomorphism restriction. (And yes, many people *do* consider the
> monomorphism restriction of standard Haskell to be a bug in the
> specification.)
>
>
When are we due a new spec? 1998 was quite a while ago...
Peter
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Message: 7
Date: Sat, 08 Jun 2013 16:15:53 -0400
From: Graham Gill <[email protected]>
Subject: Re: [Haskell-beginners] Is this a GHC bug? Problem inferring
type
To: [email protected], The Haskell-Beginners Mailing List -
Discussion of primarily beginner-level topics related to Haskell
<[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"
On 08/06/2013 4:11 PM, Peter Hall wrote:
>
>
> Show a => a -> a -> String
>
>
> ...is not monomorphic. If you want that to be inferred, turn off
> the monomorphism restriction. (And yes, many people *do* consider
> the monomorphism restriction of standard Haskell to be a bug in
> the specification.)
>
>
> When are we due a new spec? 1998 was quite a while ago...
>
> Peter
2010 <http://www.haskell.org/haskellwiki/Haskell_2010>
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