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Today's Topics:
1. FFI foreignPtr construction (PICCA Frederic-Emmanuel)
2. Re: Parsing 'A's and then ('A's or 'B's) (Francesco Ariis)
3. Re: Custom type classes (Guillaume Bouchard)
4. Re: Ambigous type variable, why this error? (David McBride)
5. Re: Parsing 'A's and then ('A's or 'B's) (Ulrik Rasmussen)
----------------------------------------------------------------------
Message: 1
Date: Wed, 27 Jan 2016 12:50:17 +0000
From: PICCA Frederic-Emmanuel
<[email protected]>
To: "[email protected]" <[email protected]>
Subject: [Haskell-beginners] FFI foreignPtr construction
Message-ID:
<a2a20ec3b8560d408356cac2fc148e53b3020...@sun-dag3.synchrotron-soleil.fr>
Content-Type: text/plain; charset="Windows-1252"
Hello,
since the last time I think that I understand how to manage Ptr, now I woud
like to masterize the ForeignPtr in order to let haskell managed the life of my
C objects.
So I try to create a Geoemtry object like this.
-- data Geometry
newtype Geometry = Geometry (Ptr Geometry) deriving (Show, Storable)
newGeometry :: Factory -> ForeignPtr Geometry
newGeometry f = unsafePerformIO $ do
geometry <- c_hkl_factory_create_new_geometry f
newForeignPtr c_hkl_geometry_free geometry
foreign import ccall safe "hkl.h hkl_factory_create_new_geometry"
c_hkl_factory_create_new_geometry :: Factory
-> IO (Geometry)
foreign import ccall safe "hkl.h &hkl_geometry_free"
c_hkl_geometry_free :: FunPtr (Geometry -> IO ())
the C signature are
HKLAPI HklGeometry *hkl_factory_create_new_geometry(const HklFactory *self)
HKL_ARG_NONNULL(1);
HKLAPI void hkl_geometry_free(HklGeometry *self) HKL_ARG_NONNULL(1);
But when I try to compile this code, I get this error message
1 of 1] Compiling Hkl.C ( src/Hkl/C.hs, dist/build/Hkl/C.o )
src/Hkl/C.hs:51:33:
Couldn't match type ?Geometry? with ?Ptr Geometry?
Expected type: GHC.ForeignPtr.FinalizerPtr Geometry
Actual type: FunPtr (Geometry -> IO ())
In the first argument of ?newForeignPtr?, namely
?c_hkl_geometry_free?
In a stmt of a 'do' block:
newForeignPtr c_hkl_geometry_free geometry
src/Hkl/C.hs:51:53:
Couldn't match expected type ?Ptr Geometry?
with actual type ?Geometry?
In the second argument of ?newForeignPtr?, namely ?geometry?
In a stmt of a 'do' block:
newForeignPtr c_hkl_geometry_free geometry
I do not understand what is wrong in my code
thanks if you can help
Frederic
------------------------------
Message: 2
Date: Wed, 27 Jan 2016 13:48:55 +0100
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Parsing 'A's and then ('A's or 'B's)
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Wed, Jan 27, 2016 at 01:34:29PM +0100, Ulrik Rasmussen wrote:
> The language is recognized by a relatively simple DFA (attached), so the
> simplest solution (I think) is to just encode that:
>
> module Main where
>
> import Text.Parsec
> import Text.Parsec.String
>
> p :: Parser ()
> p = char 'A' >> ((char 'A' >> sA) <|> (char 'B' >> sB))
> where
> sA = (char 'A' >> sA) <|> (char 'B' >> sB) <|> return ()
> sB = (char 'B' >> sB) <|> return ()
I am probably missing something: say we have an "AAB" string, how does
this check that it is `compatible` with [x,y] or [x,x,y] or [x,y,y]
(or not compatible with [x,x,x], etc.)?
------------------------------
Message: 3
Date: Wed, 27 Jan 2016 14:28:55 +0100
From: Guillaume Bouchard <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Custom type classes
Message-ID:
<CAGh0HCC1ZmipGg47jZ=xa4ryt8d3+c+kq8h4+twxw_j9a+h...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
On Wed, Jan 27, 2016 at 11:11 AM, Imants Cekusins <[email protected]> wrote:
>> `fmap` types mean that `a` should be anything, and not something
> constrained.
>
> just wondering: is it something specific to Functor class or does this
> hold for any class declaration:
>
> (a -> b)
> is not the same as
> ... a => (a -> b)
a => (a -> b) does not really mean anything as far as I know because a
is not a constraint (i.e: a typeclass).
Perhaps you mean something such as
Constraint a => (a -> b)
> in other words, if class expects (a -> b) with any a, instance must
> not constrain a.
However I discovered the `ConstraintKinds` extension which may improve
the situation.
--
G.
------------------------------
Message: 4
Date: Wed, 27 Jan 2016 09:57:33 -0500
From: David McBride <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Ambigous type variable, why this
error?
Message-ID:
<CAN+Tr42KkGWgc8c8kYipx=spx9vy1pf1oghcvvmso3lsqbg...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
If I had to guess, it is ambiguous because there are many valid instances
it could use.
>:t (Open [undefined :: Int]) <: (undefined)
(Open [undefined :: Int]) <: (undefined) :: Bool
>:t (Open [undefined :: Crust Int]) <: (undefined)
(Open [undefined :: Crust Int]) <: (undefined) :: Bool
>:t (Open [undefined :: Crust (Crust Int)]) <: (undefined)
(Open [undefined :: Crust (Crust Int)]) <: (undefined) :: Bool
However, I do not get the same error as you do on ghc 7.10.3, so I am
unsure.
<interactive>:2:9:
No instance for (Num a0) arising from the literal ?1?
The type variable ?a0? is ambiguous
Note: there are several potential instances:
instance Num Integer -- Defined in ?GHC.Num?
instance Num Double -- Defined in ?GHC.Float?
instance Num Float -- Defined in ?GHC.Float?
...plus two others
In the expression: 1
In the first argument of ?Open?, namely ?[1, 2, 3]?
In the first argument of ?(<:)?, namely ?(Open [1, 2, 3])?
<interactive>:2:17:
No instance for (Ord a0) arising from a use of ?<:?
The type variable ?a0? is ambiguous
Note: there are several potential instances:
instance (Ord a, Ord b) => Ord (Either a b)
-- Defined in ?Data.Either?
instance forall (k :: BOX) (s :: k). Ord (Data.Proxy.Proxy s)
-- Defined in ?Data.Proxy?
instance (GHC.Arr.Ix i, Ord e) => Ord (GHC.Arr.Array i e)
-- Defined in ?GHC.Arr?
...plus 28 others
In the expression: (Open [1, 2, 3]) <: (Open ([1, 2, 4]))
In an equation for ?it?:
it = (Open [1, 2, 3]) <: (Open ([1, 2, 4]))
If I remove the Ord constraint on the Poset (Crust a) instance, It changes
from ambiguous Ord to ambiguous Eq.
On Wed, Jan 27, 2016 at 3:44 AM, martin <[email protected]> wrote:
> Hello all,
>
> here is something where I don't understand the second error:
>
> *Main> (Open [1,2,3]) <: (Open ([1,2,4]))
>
> <interactive>:94:8:
> No instance for (Num a0) arising from the literal ?1?
> The type variable ?a0? is ambiguous
> Note: there are several potential instances:
> instance Num Double -- Defined in ?GHC.Float?
> instance Num Float -- Defined in ?GHC.Float?
> instance Integral a => Num (GHC.Real.Ratio a)
> -- Defined in ?GHC.Real?
> ...plus 46 others
> In the expression: 1
> In the first argument of ?Open?, namely ?[1, 2, 3]?
> In the first argument of ?(<:)?, namely ?(Open [1, 2, 3])?
>
> Okay, I understand this one, but why this:
>
> <interactive>:94:16:
> No instance for (Poset a0) arising from a use of ?<:?
> The type variable ?a0? is ambiguous
> Note: there are several potential instances:
> instance (Eq a, Ord a, Poset a) => Poset (Crust a) -- <== yes,
> yes, yes, take this one
> -- Defined at
> /home/martin/projects/haskell/currychicken/opal/Poset.hs:83:10
> instance (Eq a, Ord a, Poset a) => Poset (PsSet a)
> -- Defined at
> /home/martin/projects/haskell/currychicken/opal/Poset.hs:50:10
> instance (Eq a, Ord a, Poset a) => Poset (PsList a)
> -- Defined at
> /home/martin/projects/haskell/currychicken/opal/Poset.hs:46:10
> ...plus one other
> In the expression: (Open [1, 2, 3]) <: (Open ([1, 2, 4]))
> In an equation for ?it?:
> it = (Open [1, 2, 3]) <: (Open ([1, 2, 4]))
>
> The operands of (<:) are clearly Crusts, so (PsSet a) or (PsList a)
> shouldn't be options
>
> *Main> :t Open [1,2,3]
> Open [1,2,3] :: Num a => Crust a
> *Main>
>
> The problem goes away, when I make sure my list elements are Ints
>
> *Main> (Open [1::Int,2,3]) <: (Open ([1,2,4]))
> False
>
> But why do I see the second error at all?
>
>
> Here is the complete code:
>
> {-# Language FlexibleInstances #-}
> {-# Language UndecidableInstances #-}
>
> import qualified Data.List as L
> import qualified Data.Set as S
> import Debug.Trace
> import Test.QuickCheck hiding ((==>))
>
> ------------------------------------------------------------
> class Poset p where
> ------------------------------------------------------------
> (<:) :: p -> p -> Bool
>
> instance Poset Int where (<:) = (==)
>
> ------------------------------------------------------------
> -- Alternatives
> ------------------------------------------------------------
> newtype PsList a = PsList [a]
> newtype PsSet a = PsSet (S.Set a)
>
> isSubPolist :: (Poset a) => [a] -> [a] ->Bool
> isSubPolist as bs = all includedInBs as
> where
> includedInBs a = any (a <:) bs
>
> instance (Eq a, Ord a, Poset a) => Poset (PsList a)
> where
> (PsList as) <: (PsList bs) = isSubPolist as bs
>
> instance (Eq a, Ord a, Poset a) => Poset (PsSet a)
> where
> (PsSet as) <: (PsSet bs) = isSubPolist (S.toList as) (S.toList
> bs)
>
>
> ------------------------------------------------------------
> data Crust a = Open [a] | Closed [a]
> ------------------------------------------------------------
> deriving (Eq, Ord, Show)
>
> instance (Eq a, Ord a, Poset a) => Poset (Crust a)
> where
> (<:) (Open as) (Closed bs) = False
> (<:) (Closed as) (Closed bs) = as == bs
>
> (<:) (Open _) (Open []) = True
> (<:) (Open []) (Open _) = False
> (<:) (Open (x:xs)) (Open (y:ys)) = x <: y &&
> (Open xs) <: (Open ys)
>
> (<:) (Closed _) (Open []) = True
> (<:) (Closed []) (Open _) = False
> (<:) (Closed (x:xs)) (Open (y:ys)) = x <: y &&
> (Closed xs) <: (Open ys)
>
>
> _______________________________________________
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------------------------------
Message: 5
Date: Wed, 27 Jan 2016 16:11:23 +0100
From: Ulrik Rasmussen <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Parsing 'A's and then ('A's or 'B's)
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed
On 2016-01-27 13:48, Francesco Ariis wrote:
> On Wed, Jan 27, 2016 at 01:34:29PM +0100, Ulrik Rasmussen wrote:
> > The language is recognized by a relatively simple DFA (attached), so the
> > simplest solution (I think) is to just encode that:
> >
> > module Main where
> >
> > import Text.Parsec
> > import Text.Parsec.String
> >
> > p :: Parser ()
> > p = char 'A' >> ((char 'A' >> sA) <|> (char 'B' >> sB))
> > where
> > sA = (char 'A' >> sA) <|> (char 'B' >> sB) <|> return ()
> > sB = (char 'B' >> sB) <|> return ()
>
> I am probably missing something: say we have an "AAB" string, how does
> this check that it is `compatible` with [x,y] or [x,x,y] or [x,y,y]
> (or not compatible with [x,x,x], etc.)?
>
Oh, I read Simon's question as being constrained to the specific problem
[x, y] (i.e. recognizing AA*(AA* + BB*)).
If the problem is to run any list of parsers such as [x,y,x,x], then
this won't work.
------------------------------
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