[Haskell-cafe] Solved but strange error in type inference
Hi all, I am an Haskell newbie; can someone explain me why there is
no reported error in @legSome@ but there is one in @legSomeb@
(I used leksah as an IDE, and my compiler is:
$ ghc -v
Glasgow Haskell Compiler, Version 7.2.1, stage 2 booted by GHC version
6.12.3 )
What I do not understand is that the only difference was a typing
anotation to help the type inference, but I believed that this
annotation was already given by the signature I give, so I am quite
lost.
Thanks in advance!
==
{-# OPTIONS_GHC -XScopedTypeVariables #-}
-- why isn't this option always enabled...
{-# OPTIONS_GHC -XGADTs #-}
import Data.Word
import qualified Data.Map as M
import qualified Data.Set as S
data Symbols nt t = NT nt -- ^ non terminal
| T t -- ^ terminal
deriving (Eq, Ord)
type Sem s = [s]-s
data Rule nt t s = Rule { refined :: nt
, expression :: [Symbols nt t]
, emit :: Sem s
}
type RRule nt t s = ([Symbols nt t], Sem s)
data LegGram nt t s = Ord nt = LegGram (M.Map nt [RRule nt t s])
legSome :: LegGram nt t s - nt - Either String ([t], s)
-- ^^
--isn't this redundant?
--vv
legSome ((LegGram g)::LegGram nt t s) ntV =
case M.lookup ntV g of
Nothing - Left No word accepted!
Just l - let sg = legSome (LegGram (M.delete ntV g))
subsome :: [RRule nt t s] - Either String ([t], s)
subsome [] = Left No word accepted!
subsome ((r,sem):l) =
let makeWord [] = Right ([],[])
makeWord ((NT nnt):ll) =
do (m, ss) - sg nnt
(mm, sss) - makeWord ll
return (m++mm, ss:sss)
makeWord ((T tt):ll) =
do (mm, sss) - makeWord ll
return (tt:mm, sss)
in
case makeWord r of
Right (ll, mm) - Right (ll, sem mm)
Left err - subsome l
in subsome l
legSomeb :: LegGram nt t s - nt - Either String ([t], s)
-- but without it I have an error reported
legSomeb (LegGram g) ntV =
case M.lookup ntV g of
Nothing - Left No word accepted!
Just l - let sg = legSomeb (LegGram (M.delete ntV g))
subsome :: [RRule nt t s] - Either String ([t], s)
subsome [] = Left No word accepted!
subsome ((r,sem):l) =
let makeWord [] = Right ([],[])
makeWord ((NT nnt):ll) =
do (m, ss) - sg nnt
(mm, sss) - makeWord ll
return (m++mm, ss:sss)
makeWord ((T tt):ll) =
do (mm, sss) - makeWord ll
return (tt:mm, sss)
in
case makeWord r of
Right (ll, mm) - Right (ll, sem mm)
Left err - subsome l
in subsome l
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Re: [Haskell-cafe] Solved but strange error in type inference
Perhaps you should give us the error the compiler give you.
Plus:
data LegGram nt t s = Ord nt = LegGram (M.Map nt [RRule nt t s])
will become invalid. Currently, such class constraints are ignored.
You should remove the 'Ord nt' constraint and add it to you legSome
function. (Maybe that's a track to solve your problem...)
You have also another solution: make your LegGram type available *for
all*Ord nt (with GADTs or ExistentialQuantification), thus making you
unable to
know which type 'nt' exactly is:
data LegGram t s = forall nt. Ord nt = LegGram (M.Map nt [RRule nt t s])
or
data LegGram t s where
LegGram :: Ord nt = M.Map nt [RRule nt t s] - LegGram t s
should be both valid. I tend to prefer the latter (the use of a GADT), as
it makes you declare and handle your type constructor just like any
function.
But I don't know if it fits you requirements.
2012/1/3 AUGER Cédric sedri...@gmail.com
Hi all, I am an Haskell newbie; can someone explain me why there is
no reported error in @legSome@ but there is one in @legSomeb@
(I used leksah as an IDE, and my compiler is:
$ ghc -v
Glasgow Haskell Compiler, Version 7.2.1, stage 2 booted by GHC version
6.12.3 )
What I do not understand is that the only difference was a typing
anotation to help the type inference, but I believed that this
annotation was already given by the signature I give, so I am quite
lost.
Thanks in advance!
==
{-# OPTIONS_GHC -XScopedTypeVariables #-}
-- why isn't this option always enabled...
{-# OPTIONS_GHC -XGADTs #-}
import Data.Word
import qualified Data.Map as M
import qualified Data.Set as S
data Symbols nt t = NT nt -- ^ non terminal
| T t -- ^ terminal
deriving (Eq, Ord)
type Sem s = [s]-s
data Rule nt t s = Rule { refined :: nt
, expression :: [Symbols nt t]
, emit :: Sem s
}
type RRule nt t s = ([Symbols nt t], Sem s)
data LegGram nt t s = Ord nt = LegGram (M.Map nt [RRule nt t s])
legSome :: LegGram nt t s - nt - Either String ([t], s)
-- ^^
--isn't this redundant?
--vv
legSome ((LegGram g)::LegGram nt t s) ntV =
case M.lookup ntV g of
Nothing - Left No word accepted!
Just l - let sg = legSome (LegGram (M.delete ntV g))
subsome :: [RRule nt t s] - Either String ([t], s)
subsome [] = Left No word accepted!
subsome ((r,sem):l) =
let makeWord [] = Right ([],[])
makeWord ((NT nnt):ll) =
do (m, ss) - sg nnt
(mm, sss) - makeWord ll
return (m++mm, ss:sss)
makeWord ((T tt):ll) =
do (mm, sss) - makeWord ll
return (tt:mm, sss)
in
case makeWord r of
Right (ll, mm) - Right (ll, sem mm)
Left err - subsome l
in subsome l
legSomeb :: LegGram nt t s - nt - Either String ([t], s)
-- but without it I have an error reported
legSomeb (LegGram g) ntV =
case M.lookup ntV g of
Nothing - Left No word accepted!
Just l - let sg = legSomeb (LegGram (M.delete ntV g))
subsome :: [RRule nt t s] - Either String ([t], s)
subsome [] = Left No word accepted!
subsome ((r,sem):l) =
let makeWord [] = Right ([],[])
makeWord ((NT nnt):ll) =
do (m, ss) - sg nnt
(mm, sss) - makeWord ll
return (m++mm, ss:sss)
makeWord ((T tt):ll) =
do (mm, sss) - makeWord ll
return (tt:mm, sss)
in
case makeWord r of
Right (ll, mm) - Right (ll, sem mm)
Left err - subsome l
in subsome l
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Re: [Haskell-cafe] Solved but strange error in type inference
On Tue, Jan 3, 2012 at 5:43 PM, AUGER Cédric sedri...@gmail.com wrote: legSomeb :: LegGram nt t s - nt - Either String ([t], s) -- but without it I have an error reported legSomeb (LegGram g) ntV = case M.lookup ntV g of Nothing - Left No word accepted! Just l - let sg = legSomeb (LegGram (M.delete ntV g)) subsome :: [RRule nt t s] - Either String ([t], s) subsome [] = Left No word accepted! subsome ((r,sem):l) = let makeWord [] = Right ([],[]) makeWord ((NT nnt):ll) = do (m, ss) - sg nnt (mm, sss) - makeWord ll return (m++mm, ss:sss) makeWord ((T tt):ll) = do (mm, sss) - makeWord ll return (tt:mm, sss) in case makeWord r of Right (ll, mm) - Right (ll, sem mm) Left err - subsome l in subsome l I found it compiling well if removing this line: subsome :: [RRule nt t s] - Either String ([t], s) It seems to me that the compiler is not sure the two 'nt' are equal. The ScopedTypeVariables can make the compiler believe they are equal. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
On Tue, Jan 3, 2012 at 05:17, Yucheng Zhang yczhan...@gmail.com wrote: subsome :: [RRule nt t s] - Either String ([t], s) It seems to me that the compiler is not sure the two 'nt' are equal. The ScopedTypeVariables can make the compiler believe they are equal. But ScopedTypeVariables is enabled already. OTOH, I *think* the embedded Ord constraint is screwing things up (which is why it's showing up in the error message), because it's embedded and therefore has its own limited scope which prevents ScopedTypeVariables from doing the right thing. This idiosyncrasy is why those embedded constraints are being removed from the language except in more controllable and useful ways (namely GADTs; the H98 version is not capable of doing anything useful, as I understand it, it can only screw up typing in situations like this without actually adding anything useful to the type system, as it's not possible to bring the Ord constraint usefully into scope outside the data declaration itself). (More technically, and I think this is correct: the effective data type is (forall nt. Ord nt = LegGram (M.Map nt [RRule nt t s])) Note the outer parentheses; the Ord constraint is *only* active within those outer parentheses, meaning that it prevents the type of nt from matching that in any other use of the type, even in the same expression, even with ScopedTypeVariables.) -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
On Tue, Jan 3, 2012 at 6:44 PM, Brandon Allbery allber...@gmail.com wrote: On Tue, Jan 3, 2012 at 05:17, Yucheng Zhang yczhan...@gmail.com wrote: subsome :: [RRule nt t s] - Either String ([t], s) It seems to me that the compiler is not sure the two 'nt' are equal. The ScopedTypeVariables can make the compiler believe they are equal. But ScopedTypeVariables is enabled already. Sorry, I meant actually using ScopedTypeVariables as in the first function, which compiles well: legSome :: LegGram nt t s - nt - Either String ([t], s) -- ^^ --isn't this redundant? --vv legSome ((LegGram g)::LegGram nt t s) ntV = case M.lookup ntV g of Nothing - Left No word accepted! Just l - let sg = legSome (LegGram (M.delete ntV g)) subsome :: [RRule nt t s] - Either String ([t], s) subsome [] = Left No word accepted! subsome ((r,sem):l) = let makeWord [] = Right ([],[]) makeWord ((NT nnt):ll) = do (m, ss) - sg nnt (mm, sss) - makeWord ll return (m++mm, ss:sss) makeWord ((T tt):ll) = do (mm, sss) - makeWord ll return (tt:mm, sss) in case makeWord r of Right (ll, mm) - Right (ll, sem mm) Left err - subsome l in subsome l ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
On Tue, Jan 3, 2012 at 05:50, Yucheng Zhang yczhan...@gmail.com wrote: On Tue, Jan 3, 2012 at 6:44 PM, Brandon Allbery allber...@gmail.com wrote: On Tue, Jan 3, 2012 at 05:17, Yucheng Zhang yczhan...@gmail.com wrote: subsome :: [RRule nt t s] - Either String ([t], s) It seems to me that the compiler is not sure the two 'nt' are equal. The ScopedTypeVariables can make the compiler believe they are equal. But ScopedTypeVariables is enabled already. Sorry, I meant actually using ScopedTypeVariables as in the first function, which compiles well: legSome :: LegGram nt t s - nt - Either String ([t], s) Except he's not; an explicit forall is needed to indicate the type variables to be brought into scope. It would need to be legSome :: forall nt t s. LegGram nt t s - nt - Either String ([t], s) -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
As I understand it, both ways work. legSome ((LegGram g)::LegGram nt t s) ntV If you compile this without ScopedTypeVariables extension, GHC will remind you of it: Illegal signature in pattern: LegGram nt t s Use -XScopedTypeVariables to permit it So another solution is to avoid the ugly redundancy is: legSome :: forall nt t s . LegGram nt t s - nt - Either String ([t], s) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
That is error-prone. Plus the code does not need ScopedTypeVariables. The real problem comes from the use of a class constraint on the LegGram data constructor. data LegGram nt t s = Ord nt = LegGram (M.Map nt [RRule nt t s]) Short answer: you *can't *add class constraints to an already declared type variable: The LegGram *type *constructor declares the 'nt' variable (and then brings it into scope), so trying afterwards to add a constraint to it for the LegGram *data *constructor is invalid, so the compiler understands this: data LegGram nt t s = *forall nt.* Ord nt = LegGram (M.Map *nt* [RRule *nt*t s]) ...the declaration and scoping of a *new* type variable. This is it, right? 2012/1/3 Yucheng Zhang yczhan...@gmail.com As I understand it, both ways work. legSome ((LegGram g)::LegGram nt t s) ntV If you compile this without ScopedTypeVariables extension, GHC will remind you of it: Illegal signature in pattern: LegGram nt t s Use -XScopedTypeVariables to permit it So another solution is to avoid the ugly redundancy is: legSome :: forall nt t s . LegGram nt t s - nt - Either String ([t], s) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
On Tue, Jan 3, 2012 at 06:35, Yucheng Zhang yczhan...@gmail.com wrote: legSome ((LegGram g)::LegGram nt t s) ntV If you compile this without ScopedTypeVariables extension, GHC will remind you of it: Illegal signature in pattern: LegGram nt t s Use -XScopedTypeVariables to permit it Right, but I think this is conflating two aspects of ScopedTypeVariables and may not bring them into scope fully. Although, that's a question for someone who understands ghc's type system far better than I do. -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
On Tue, Jan 3, 2012 at 7:46 PM, Brandon Allbery allber...@gmail.com wrote: Right, but I think this is conflating two aspects of ScopedTypeVariables and may not bring them into scope fully. Although, that's a question for someone who understands ghc's type system far better than I do. I found some descriptions of ScopedTypeVariables here: http://hackage.haskell.org/trac/haskell-prime/wiki/ScopedTypeVariables ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
On Tue, Jan 3, 2012 at 06:43, Yves Parès limestr...@gmail.com wrote: That is error-prone. Plus the code does not need ScopedTypeVariables. The real problem comes from the use of a class constraint on the LegGram data constructor. data LegGram nt t s = Ord nt = LegGram (M.Map nt [RRule nt t s]) Short answer: you *can't *add class constraints to an already declared type variable: The LegGram *type *constructor declares the 'nt' variable (and then brings it into scope), so trying afterwards to add a constraint to it for the LegGram *data *constructor is invalid, so the compiler understands this: data LegGram nt t s = *forall nt.* Ord nt = LegGram (M.Map *nt* [RRule * nt* t s]) ...the declaration and scoping of a *new* type variable. This is it, right? Yep, with the additional gotcha that two uses of the same data constructor in an expression will not unify those type variables because it's scoped to the data constructor itself. Which is the confusing and error-prone behavior that led to it being removed from the next version of the standard (probably; the proposal was accepted but H'2011 was bypassed and H'2012 has not yet started). -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
On Tue, Jan 3, 2012 at 7:49 PM, Yucheng Zhang yczhan...@gmail.com wrote: I found some descriptions of ScopedTypeVariables here: http://hackage.haskell.org/trac/haskell-prime/wiki/ScopedTypeVariables Sorry, I found just now a more up-to-date description in the GHC documentation: http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#scoped-type-variables ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
Thanks all, I finally give up to put Ord in the LegGram type. What was annoying me was that @legsome@ was in fact an instance of a class I defined. So I changed its signature to make it depend on Ord. That is not very nice, since at first glance, there could be implementations which does not require nt to be ordered. But as in some other method of this class I need it, I guess it isn't that annoying. I still do find the reported error strange, but thanks to your remarks, I can remove some typing annotation in the body of the function (which I forgot to remove and used while I was debugging my types) to avoid this ugly redundancy (I guess I could also remove the global type signature, but I like to keep them). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
The other much simpler solution no one has mentioned yet is to just
pull 'subsome' out as its own top-level declaration. Having such a
big function nested locally within a 'let' is ugly anyway, and it
makes it harder to test and debug than necessary.
-Brent
On Tue, Jan 03, 2012 at 05:44:01PM +0100, Yves Parès wrote:
Remove subsome type signature. You are redeclaring type variables which
obviously cannot match those of legSome.
This cannot work without scoped type variables (and ad-hoc foralls to bring
them to scope, of course).
2012/1/3 Yucheng Zhang yczhan...@gmail.com
As I investigated the code more carefully, I found that the type
unification
failure may not be related to the suspected class constraint on data
constructor.
I have made minor changes to the original code to remove the Ord
constraint,
including introducing a FakedMap with no requirement on Ord. The type
unification
failure continues:
Couldn't match type `nt1' with `nt'
`nt1' is a rigid type variable bound by
the type signature for
subsome :: [RRule nt1 t1 s1] - Either String ([t1], s1)
at xx.hs:34:19
`nt' is a rigid type variable bound by
the type signature for
legSome :: LegGram nt t s - nt - Either String ([t], s)
at xx.hs:29:1
Expected type: [Symbols nt1 t1]
Actual type: [Symbols nt t]
In the first argument of `makeWord', namely `r'
In the expression: makeWord r
The complete changed code follows:
data Symbols nt t = NT nt -- ^ non terminal
| T t -- ^ terminal
deriving (Eq, Ord)
type Sem s = [s]-s
data Rule nt t s = Rule { refined :: nt
, expression :: [Symbols nt t]
, emit :: Sem s
}
type RRule nt t s = ([Symbols nt t], Sem s)
data FakedMap a b = FakedMap
delete :: k - FakedMap k a - FakedMap k a
delete a b = b
lookup :: k - FakedMap k a - Maybe a
lookup a b = Nothing
data LegGram nt t s = LegGram (FakedMap nt [RRule nt t s])
legSome :: LegGram nt t s - nt - Either String ([t], s)
legSome (LegGram g) ntV =
case Main.lookup ntV g of
Nothing - Left No word accepted!
Just l - let sg = legSome (LegGram (Main.delete ntV g))
subsome :: [RRule nt t s] - Either String ([t], s)
subsome [] = Left No word accepted!
subsome ((r,sem):l) =
let makeWord [] = Right ([],[])
makeWord ((NT nnt):ll) =
do (m, ss) - sg nnt
(mm, sss) - makeWord ll
return (m++mm, ss:sss)
makeWord ((T tt):ll) =
do (mm, sss) - makeWord ll
return (tt:mm, sss)
in
case makeWord r of
Right (ll, mm) - Right (ll, sem mm)
Left err - subsome l
in subsome l
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Re: [Haskell-cafe] Solved but strange error in type inference
On Wed, Jan 4, 2012 at 12:44 AM, Yves Parès limestr...@gmail.com wrote: Remove subsome type signature. You are redeclaring type variables which obviously cannot match those of legSome. This cannot work without scoped type variables (and ad-hoc foralls to bring them to scope, of course). That subsome type signature is from the original code. I wonder why the redeclared type variables cannot match those of legSome? p.s. I just realized that my changed code missed the line: import qualified Data.Map as M ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
Brent is right. Separating functions is nicer to read and cleaner. Plus it enhances testability. I wonder why the redeclared type variables cannot match those of legSome? Try to put a totally wrong type to subsome, like subsome :: Int and tell us from the error what type is actually inferred. 2012/1/3 Yucheng Zhang yczhan...@gmail.com On Wed, Jan 4, 2012 at 12:44 AM, Yves Parès limestr...@gmail.com wrote: Remove subsome type signature. You are redeclaring type variables which obviously cannot match those of legSome. This cannot work without scoped type variables (and ad-hoc foralls to bring them to scope, of course). That subsome type signature is from the original code. I wonder why the redeclared type variables cannot match those of legSome? p.s. I just realized that my changed code missed the line: import qualified Data.Map as M ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
On Wed, Jan 4, 2012 at 1:07 AM, Yves Parès limestr...@gmail.com wrote: Try to put a totally wrong type to subsome, like subsome :: Int and tell us from the error what type is actually inferred. The error is like Couldn't match type `nt' with `Int' `nt' is a rigid type variable bound by the type signature for legSome :: LegGram nt t s - nt - Either String ([t], s) at xx.hs:35:1 Expected type: [RRule Int t s] Actual type: [RRule nt t s] In the first argument of `subsome', namely `l' In the expression: subsome l However, this error cannot be resolved by a use of ScopedTypeVariable, while the original error can be resolved. I still don't see where the mismatch happens in the original error. Any help is appreciated. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
On Wed, Jan 4, 2012 at 1:38 AM, Yucheng Zhang yczhan...@gmail.com wrote: On Wed, Jan 4, 2012 at 1:07 AM, Yves Parès limestr...@gmail.com wrote: Try to put a totally wrong type to subsome, like subsome :: Int and tell us from the error what type is actually inferred. Sorry, I found I misunderstood the suggestion. I will try. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
On Wed, Jan 4, 2012 at 1:07 AM, Yves Parès limestr...@gmail.com wrote: I wonder why the redeclared type variables cannot match those of legSome? Try to put a totally wrong type to subsome, like subsome :: Int and tell us from the error what type is actually inferred. The actually inferred type is Couldn't match expected type `Int' with actual type `[([Symbols nt t], [s] - t0)] - Either [Char] ([t], t0)' In the expression: subsome :: Int I still don't understand why the original mismatch happens. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
2012/1/3 Yucheng Zhangyczhan...@gmail.com (Hopefully being a little more explicit about this can help you understand where things are going wrong.) [--snip--] legSome :: LegGram nt t s - nt - Either String ([t], s) The 'nt' you see above legSome (LegGram g) ntV = case Main.lookup ntV g of Nothing - Left No word accepted! Just l - let sg = legSome (LegGram (Main.delete ntV g)) subsome :: [RRule nt t s] - Either String ([t], s) ... isn't the same as the 'nt' you see in this line, so it constrains 'subsome' to a different type than the one you intended -- and indeed one which can't be unified with the inferred type. (Unless you use ScopedTypeVariables.) As Brent suggested, you should probably pull the subsome function out into a top-level function in any case. Cheers, ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
On Wed, Jan 4, 2012 at 2:48 AM, Bardur Arantsson s...@scientician.net wrote: 'subsome' to a different type than the one you intended -- and indeed one which can't be unified with the inferred type. (Unless you use ScopedTypeVariables.) Thanks for the reply. Actually, my question is why the different type can't be unified with the inferred type? Could you point me some related resources? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Is the haddock generator on Hackage down?
Hi all, No Haddock documentation seems to have been generated on Hackage in the past few days. See e.g. http://hackage.haskell.org/package/copilot-2.0.3 or http://hackage.haskell.org/package/derive-2.5.5 Does anyone know if the cron job (or whatever) isn't running for some reason? Cheers, ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] space-efficient, composable list transformers [was: Re: Reifying case expressions]
Jan Christiansen wrote: On Jan 2, 2012, at 2:34 PM, Heinrich Apfelmus wrote: Without an explicit guarantee that the function is incremental, we can't do anything here. But we can just add another constructor to that effect if we turn ListTo into a GADT: data ListTo a b where CaseOf :: b - (a - ListTo a b) - ListTo a b Fmap :: (b - c) - ListTo a b - ListTo a c FmapCons :: b - ListTo a [b] - ListTo a [b] I did not follow your discussion but how about using an additional GADT for the argument of Fmap, that is data Fun a b where Fun :: (a - b) - Fun a b Cons :: a - Fun [a] [a] data ListTo a b where CaseOf :: b - (a - ListTo a b) - ListTo a b Fmap :: Fun b c - ListTo a b - ListTo a c and provide a function to interpret this data type as well interpretFun :: Fun a b - a - b interpretFun (Fun f) = f interpretFun (Cons x) = (x:) for the sequential composition if I am not mistaken. (.) :: ListTo b c - ListTo a [b] - ListTo a c (CaseOf _ cons) . (Fmap (Cons y) b) = cons y . b (Fmap f a) . (Fmap g b) = Fmap f $ a . (Fmap g b) a . (CaseOf nil cons)= CaseOf (interpret a nil) ((a .) . cons) a . (Fmap f b) = fmap (interpret a . interpretFun f) b -- functor instance instance Functor (ListTo a) where fmap f = normalize . Fmap (Fun f) normalize :: ListTo a b - ListTo a b normalize (Fmap (Fun f) (Fmap (Fun g) c)) = fmap (f . g) c normalize x = x Cheers, Jan Nice, that is a lot simpler indeed, and even closer to the core of the idea. 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] Is the haddock generator on Hackage down?
On Tue, Jan 03, 2012 at 07:14:58PM +, Bardur Arantsson wrote: No Haddock documentation seems to have been generated on Hackage in the past few days. The machine's down, I think. I'll give it a kick tomorrow. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Experiments in Haskell Packaging
Hi Ryan, Ryan Grant [mailto:rgr...@rgrant.org] said: Are packages cabal-installed using --global? The packages that come with a compiler or Haskell Platform are installed --global. Every other package is installed with cabal, by default into the user's space. Is it mostly reliant on cabal or cabal-dev? It works with cabal. I have not tested interworking with cabal-dev yet. (There is a big overlap in the functionality of course.) How easily can I upgrade cabal? http://stackoverflow.com/questions/5380888/cabal-install-and-debian The stackoverflow discussion seems a little confused in that 'cabal update' just updates the local cache of package headers from the server, not any of the installed packages. But I am using the latest version of cabal-install (0.10.2), compiled with cabal-1.10.1.0 library (from March rather than June's cabal-1.10.2.0). Users should never have to build their own cabal-install, but just keep up to date with the distro. If users should need to use their own private Cabal builds for whatever reason we will provide a wiki page showing them how to do it. Can the idea be extended to OSX, as well? I see no reason why not. The idea behind my post was really to say that we should be doing this with all distros. I will be making the hub sources available through hackage/github when beta testing is complete (but sooner if demand dictates). Folks can install it on their own systems and hook in their own GHC and HP builds. Best of all folks can make it available in other Haskell distros. The interesting question is whether anyone else decides to structure their distros in the same way as the justhub distro. That is the real challenge! How does this strategy relate to the Nix package manager? http://nixos.org/nix/ In a sense the philosophy is similar because I am arguing that we shouldn't be updating distributions, just adding to them. In that sense it is a functional philosophy. But after that 30,000 foot observation the comparison diverges as far as I can see. Great questions! Thanks, Chris ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solved but strange error in type inference
Actually, my question is why the different type can't be unified with the inferred type? Because without ScopedTypeVariable, both types got expanded to : legSome :: *forall nt* t s. LegGram nt t s - nt - Either String ([t], s) subsome :: *forall nt* t s. [RRule nt t s] - Either String ([t], s) So you see subsome declare new variables, which *do not *match the *rigid *ones declared by legSome signature, hence the incompatibility. As we said before, you have three ways to work it out: 1) Use scoped type variables with explicit forall nt on legSome and *no forall *on subsome, so that nt in subsome matches nt declared in legSome. 2) Don't give a type signature for subsome and let GHC find out which is its correct type. 3) Extract subsome so that it becomes a top-level function with a polymorphic type (Recommended). Now, concerning the error I asked you to deliberately provoke, that's the quickest way I found to know what is the output of the type inferer, and maybe the only simple one. So this error is: Couldn't match expected type `Int' with actual type `[([Symbols nt t], [s] - t0)] - Either [Char] ([t], t0)' In the expression: subsome :: Int GHC tells you the type it infers for subsome: [([Symbols nt t], [s] - t0)] - Either [Char] ([t], t0) The nt you see is the one from legSome, those messages show scoped variables. (You'd get something like 'nt1' or 'nt0' if it was another variable, meant to be instanciated to a different type). This accords with your previous error message, which happens to give you more details about where the rigid type variable nt comes from: `nt' is a rigid type variable bound by the type signature for legSome :: LegGram nt t s - nt - Either String ([t], s) HTH. 2012/1/3 Yucheng Zhang yczhan...@gmail.com On Wed, Jan 4, 2012 at 2:48 AM, Bardur Arantsson s...@scientician.net wrote: 'subsome' to a different type than the one you intended -- and indeed one which can't be unified with the inferred type. (Unless you use ScopedTypeVariables.) Thanks for the reply. Actually, my question is why the different type can't be unified with the inferred type? Could you point me some related resources? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to split this string.
If you're interested in learning parsec, RWH covered this topic in depth in Chapter 16, Choices and Errors: http://book.realworldhaskell.org/read/using-parsec.html. On Mon, Jan 2, 2012 at 3:44 AM, max m...@mtw.ru wrote: I want to write a function whose behavior is as follows: foo string1\nstring2\r\nstring3\nstring4 = [string1, string2\r\nstring3, string4] Note the sequence \r\n, which is ignored. How can I do this? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] GHC compilation issue: Working around bug #4374?
I am getting the compilation error below when building GHC 7.0.4 on RHEL5
(x86_64) – I configured make to point to ncurses and libgmp. I will appreciated
pointers on how to work around the issue below. I don’t have root privileges.
My apologies if this has already been discussed in this forum. I searched for
the issue but could find only the GHC bug ticket below.
GHC bug ticket: http://hackage.haskell.org/trac/ghc/ticket/4374 - it doesn’t
have description on how to work around the bug.
Compilation error:
---
$
C_INCLUDE_PATH=/efs/dist/fsf/ncurses/5.6/common/include/ncurses:/home/saagrawa/scripts/misc/libgmp/include
LIBRARY_PATH=/efs/dist/fsf/ncurses/5.6/exec/lib:/home/saagrawa/scripts/misc/libgmp/lib
make
===--- updating makefiles phase 0
make -r --no-print-directory -f ghc.mk phase=0 just-makefiles
===--- updating makefiles phase 1
make -r --no-print-directory -f ghc.mk phase=1 just-makefiles
===--- updating makefiles phase 2
make -r --no-print-directory -f ghc.mk phase=2 just-makefiles
===--- updating makefiles phase 3
make -r --no-print-directory -f ghc.mk phase=3 just-makefiles
===--- finished updating makefiles
make -r --no-print-directory -f ghc.mk all
inplace/bin/ghc-stage1
libraries/integer-gmp/dist-install/build/GHC/Integer.dyn_o
libraries/integer-gmp/dist-install/build/GHC/Integer/GMP/Internals.dyn_o
libraries/integer-gmp/dist-install/build/GHC/Integer/Type.dyn_o
libraries/integer-gmp/dist-install/build/cbits/gmp-wrappers.dyn_o
libraries/integer-gmp/dist-install/build/cbits/cbits.dyn_o
libraries/integer-gmp/gmp/objs/*.o `/usr/bin/find
libraries/integer-gmp/dist-install/build -name *_stub.dyn_o -print` -shared
-dynamic -dynload deploy -dylib-install-name
/home/saagrawa/scripts/misc/ghc/7.0.4/lib/ghc-7.0.4/`basename
libraries/integer-gmp/dist-install/build/libHSinteger-gmp-0.2.0.3-ghc7.0.4.so
| sed 's/^libHS//;s/[-]ghc.*//'`/`basename
libraries/integer-gmp/dist-install/build/libHSinteger-gmp-0.2.0.3-ghc7.0.4.so`
-no-auto-link-packages -package ghc-prim-0.2.0.0 -o
libraries/integer-gmp/dist-install/build/libHSinteger-gmp-0.2.0.3-ghc7.0.4.so
/efs/dist/fsf/gcc/4.4.3-build004/.exec/x86-64.rhel.5/libexec/gcc/x86_64-unknown-linux-gnu/4.4.3/ld:
libraries/integer-gmp/gmp/objs/aors.o: relocation R_X86_64_32 against
`__gmpz_sub' can not be used when making a shared object; recompile with -fPIC
libraries/integer-gmp/gmp/objs/aors.o: could not read symbols: Bad value
collect2: ld returned 1 exit status
make[1]: ***
[libraries/integer-gmp/dist-install/build/libHSinteger-gmp-0.2.0.3-ghc7.0.4.so]
Error 1
make: *** [all] Error 2
$ ll /home/saagrawa/scripts/misc/libgmp/lib|awk '{print $9,$10,$11}' ### the
libgmp lib folder has dynamic libraries
libgmp.so.3.5.2*
libgmp.so.3 - libgmp.so.3.5.2*
libgmp.so - libgmp.so.3.5.2*
libgmp.la*
libgmp.a___
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Re: [Haskell-cafe] GHC compilation issue: Working around bug #4374?
My apologies for posting it here. Glasgow-haskell-users mailing list seems
more relevant for this question. So, I am going to send it there instead,
with a note about this cross-posting.
On Tue, Jan 3, 2012 at 4:26 PM, Sanket Agrawal sanket.agra...@gmail.comwrote:
I am getting the compilation error below when building GHC 7.0.4 on RHEL5
(x86_64) – I configured make to point to ncurses and libgmp. I will
appreciated pointers on how to work around the issue below. I don’t have
root privileges. My apologies if this has already been discussed in this
forum. I searched for the issue but could find only the GHC bug ticket
below.
GHC bug ticket: http://hackage.haskell.org/trac/ghc/ticket/4374 - it
doesn’t have description on how to work around the bug.
Compilation error:
---
$
C_INCLUDE_PATH=/efs/dist/fsf/ncurses/5.6/common/include/ncurses:/home/saagrawa/scripts/misc/libgmp/include
LIBRARY_PATH=/efs/dist/fsf/ncurses/5.6/exec/lib:/home/saagrawa/scripts/misc/libgmp/lib
make
===--- updating makefiles phase 0
make -r --no-print-directory -f ghc.mk phase=0 just-makefiles
===--- updating makefiles phase 1
make -r --no-print-directory -f ghc.mk phase=1 just-makefiles
===--- updating makefiles phase 2
make -r --no-print-directory -f ghc.mk phase=2 just-makefiles
===--- updating makefiles phase 3
make -r --no-print-directory -f ghc.mk phase=3 just-makefiles
===--- finished updating makefiles
make -r --no-print-directory -f ghc.mk all
inplace/bin/ghc-stage1
libraries/integer-gmp/dist-install/build/GHC/Integer.dyn_o
libraries/integer-gmp/dist-install/build/GHC/Integer/GMP/Internals.dyn_o
libraries/integer-gmp/dist-install/build/GHC/Integer/Type.dyn_o
libraries/integer-gmp/dist-install/build/cbits/gmp-wrappers.dyn_o
libraries/integer-gmp/dist-install/build/cbits/cbits.dyn_o
libraries/integer-gmp/gmp/objs/*.o `/usr/bin/find
libraries/integer-gmp/dist-install/build -name *_stub.dyn_o -print`
-shared -dynamic -dynload deploy -dylib-install-name
/home/saagrawa/scripts/misc/ghc/7.0.4/lib/ghc-7.0.4/`basename
libraries/integer-gmp/dist-install/build/
libHSinteger-gmp-0.2.0.3-ghc7.0.4.so | sed
's/^libHS//;s/[-]ghc.*//'`/`basename
libraries/integer-gmp/dist-install/build/
libHSinteger-gmp-0.2.0.3-ghc7.0.4.so` -no-auto-link-packages -package
ghc-prim-0.2.0.0 -o libraries/integer-gmp/dist-install/build/
libHSinteger-gmp-0.2.0.3-ghc7.0.4.so
/efs/dist/fsf/gcc/4.4.3-build004/.exec/x86-64.rhel.5/libexec/gcc/x86_64-unknown-linux-gnu/4.4.3/ld:
libraries/integer-gmp/gmp/objs/aors.o: relocation R_X86_64_32 against
`__gmpz_sub' can not be used when making a shared object; recompile with
-fPIC
libraries/integer-gmp/gmp/objs/aors.o: could not read symbols: Bad value
collect2: ld returned 1 exit status
make[1]: *** [libraries/integer-gmp/dist-install/build/
libHSinteger-gmp-0.2.0.3-ghc7.0.4.so] Error 1
make: *** [all] Error 2
$ ll /home/saagrawa/scripts/misc/libgmp/lib|awk '{print $9,$10,$11}' ###
the libgmp lib folder has dynamic libraries
**
**
libgmp.so.3.5.2*
libgmp.so.3 - libgmp.so.3.5.2*
libgmp.so - libgmp.so.3.5.2*
libgmp.la*
libgmp.a
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