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Today's Topics:
1. Re: Type inference in ST monad? (Brandon Allbery)
2. Re: Type inference in ST monad? (Aurimas)
3. multi-parameter typeclass with default implementation (TP)
4. Re: multi-parameter typeclass with default implementation
(Ben Gamari)
5. Re: How Haskell Fits Into an Operating System / API
Environment (Magnus Therning)
6. Re: multi-parameter typeclass with default implementation (TP)
7. Re: multi-parameter typeclass with default implementation (TP)
----------------------------------------------------------------------
Message: 1
Date: Mon, 19 Aug 2013 09:59:28 -0400
From: Brandon Allbery <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Type inference in ST monad?
Message-ID:
<cakfcl4uavyyq3idjsqx_wa2jmmckcuv9bexehh41yjxom7p...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Mon, Aug 19, 2013 at 1:36 AM, Aurimas <[email protected]> wrote:
> Thanks, this is clearly the most easy way to fix the problem. But the
> question remains - why the line
> "do r1 <- uniformR (-1.0, 1.0 :: Double) gen" cannot be annotated with
> ST s Double?
>
Maybe you need ScopedTypeVariables? But I'd be suspicious that, given the
forall in the definition of ST, that such a type annotation *always*
creates a new `s` and some other way to "force" the type is needed.
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 2
Date: Mon, 19 Aug 2013 17:12:27 +0300
From: Aurimas <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Type inference in ST monad?
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"
On 08/19/2013 04:59 PM, Brandon Allbery wrote:
> On Mon, Aug 19, 2013 at 1:36 AM, Aurimas <[email protected]
> <mailto:[email protected]>> wrote:
>
> Thanks, this is clearly the most easy way to fix the problem. But
> the question remains - why the line
> "do r1 <- uniformR (-1.0, 1.0 :: Double) gen" cannot be annotated
> with ST s Double?
>
>
> Maybe you need ScopedTypeVariables? But I'd be suspicious that, given
> the forall in the definition of ST, that such a type annotation
> *always* creates a new `s` and some other way to "force" the type is
> needed.
>
ScopedTypeVariables does not solve the problem. As you noticed, new s is
created and runST is not happy about this. It is
strange that this type can be "forced" replacing given line by a
function returning ST s a.
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Message: 3
Date: Mon, 19 Aug 2013 22:28:14 +0200
From: TP <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] multi-parameter typeclass with default
implementation
Message-ID: <[email protected]>
Content-Type: text/plain; charset="ISO-8859-1"
Hi,
I struggle with a dummy example using a multi-parameter typeclass containing
a default implementation for a function:
---------------------------------------
{-# LANGUAGE MultiParamTypeClasses #-}
class Foo a b where
bar :: a -> Int
foobar :: a -> b -> Int
foobar avalue bvalue = bar avalue
instance Foo Int Int where
bar i = 5
main = do
print $ bar (4::Int)
---------------------------------------
I obtain the following errors. I have tried various things without any
success.
Any help appreciated!
Thanks
TP
PS: The errors:
$ runghc test.hs
test.hs:8:28:
Could not deduce (Foo a b1) arising from a use of `bar'
from the context (Foo a b)
bound by the class declaration for `Foo' at test.hs:(3,1)-(8,37)
The type variable `b1' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
In the expression: bar avalue
In an equation for `foobar': foobar avalue bvalue = bar avalue
test.hs:16:9:
No instance for (Foo Int b0) arising from a use of `bar'
The type variable `b0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there is a potential instance available:
instance Foo Int Int -- Defined at test.hs:10:10
Possible fix: add an instance declaration for (Foo Int b0)
In the second argument of `($)', namely `bar (4 :: Int)'
In a stmt of a 'do' block: print $ bar (4 :: Int)
In the expression: do { print $ bar (4 :: Int) }
------------------------------
Message: 4
Date: Mon, 19 Aug 2013 16:59:42 -0400
From: Ben Gamari <[email protected]>
To: TP <[email protected]>, [email protected]
Subject: Re: [Haskell-beginners] multi-parameter typeclass with
default implementation
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
TP <[email protected]> writes:
> Hi,
>
> I struggle with a dummy example using a multi-parameter typeclass containing
> a default implementation for a function:
>
> ---------------------------------------
> {-# LANGUAGE MultiParamTypeClasses #-}
>
> class Foo a b where
>
> bar :: a -> Int
>
The problem is in this declaration, which does not mention the type
"b". This makes it impossible for the compiler to infer which instance
to use when "bar" is used. This is what the compiler is trying to tell
you when it says "The type variable `b1' is ambiguous".
As far as I know, you'd need to do something like this to accomplish
what you are after,
{-# LANGUAGE MultiParamTypeClasses, DefaultSignatures #-}
class Bar a where
bar :: a -> Int
class FooBar a b where
foobar :: a -> b -> Int
default foobar :: Bar a => a -> b -> Int
foobar avalue bvalue = bar avalue
instance Bar Int where
bar i = 5
instance FooBar Int Int
main = do
print $ bar (4::Int)
print $ foobar (5::Int) (2::Int)
Cheers,
- Ben
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Message: 5
Date: Tue, 20 Aug 2013 10:11:39 +0200
From: Magnus Therning <[email protected]>
To: Henk-Jan van Tuyl <[email protected]>
Cc: [email protected]
Subject: Re: [Haskell-beginners] How Haskell Fits Into an Operating
System / API Environment
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
On Mon, Aug 19, 2013 at 12:52:20PM +0200, Henk-Jan van Tuyl wrote:
> On Mon, 19 Aug 2013 10:18:11 +0200, Magnus Therning
> <[email protected]> wrote:
>
> >There are also a few articles on Erlang, and I believe there's good
> >reason to believe the results with that language can be transferred to
> >other "languages without assignment".
> >
> >- Productivity gains with Erlarng
> > http://dl.acm.org/citation.cfm?id=1362710
> > slides: http://is.gd/BgGZyh
> > (behind a paywall, I haven't found a freely available copy)
> >
>
> For free:
> "Four-fold Increase in Productivity and Quality"
> Ulf Wiger, FemSYS 2001, Munich
> http://www.erlang.se/publications/Ulf_Wiger.pdf
Thanks! I was trying to find that paper yesterday, but apparently my
google-fu isn't as strong as yours ;)
/M
--
Magnus Therning OpenPGP: 0xAB4DFBA4
email: [email protected] jabber: [email protected]
twitter: magthe http://therning.org/magnus
Most software today is very much like an Egyptian pyramid with
millions of bricks piled on top of each other, with no structural
integrity, but just done by brute force and thousands of slaves.
-- Alan Kay
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------------------------------
Message: 6
Date: Tue, 20 Aug 2013 11:44:49 +0200
From: TP <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] multi-parameter typeclass with
default implementation
Message-ID: <[email protected]>
Content-Type: text/plain; charset="UTF-8"
Ben Gamari wrote:
>> {-# LANGUAGE MultiParamTypeClasses #-}
>>
>> class Foo a b where
>>
>> bar :: a -> Int
>>
> The problem is in this declaration, which does not mention the type
> "b". This makes it impossible for the compiler to infer which instance
> to use when "bar" is used. This is what the compiler is trying to tell
> you when it says "The type variable `b1' is ambiguous".
Thanks, this works perfectly. Yet, to try to improve myself, I would like to
discuss a bit on the text of the two obtained errors in my example (correct
me if I am wrong below).
"""
test.hs:16:9:
No instance for (Foo Int b0) arising from a use of `bar'
"""
The second error is clear: indeed by calling `bar` on `4::Int`, we don't
indicate the type of b allowing to choose the right typeclass for the
implementation of the `bar` function. The fact there is only one typeclass
instance with Int as type for type variable `a` does not change anything:
Haskell does not look at the implemented instances. In other words,
?typeclasses are open?: more typeclasses with Int as first type variable
(and types different from `Int` for type variable `b`) may be added in the
future, and Haskell asks to specify which one is to use now, not in the
future. (Is this the right meaning for "typeclasses are open"?).
"""
test.hs:8:28:
Could not deduce (Foo a b1) arising from a use of `bar'
from the context (Foo a b)
bound by the class declaration for `Foo' at test.hs:(3,1)-(8,37)
The type variable `b1' is ambiguous
"""
Is this the same interpretation here? That is, even in the default
implementation of a typeclass function, Haskell does not suppose that the
instance of `bar` to use is the one of the considered instance?
Thanks a lot,
TP
------------------------------
Message: 7
Date: Tue, 20 Aug 2013 12:13:28 +0200
From: TP <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] multi-parameter typeclass with
default implementation
Message-ID: <[email protected]>
Content-Type: text/plain; charset="ISO-8859-1"
Ben Gamari wrote:
> As far as I know, you'd need to do something like this to accomplish
> what you are after,
>
> {-# LANGUAGE MultiParamTypeClasses, DefaultSignatures #-}
>
> class Bar a where
> bar :: a -> Int
>
> class FooBar a b where
> foobar :: a -> b -> Int
> default foobar :: Bar a => a -> b -> Int
> foobar avalue bvalue = bar avalue
>
> instance Bar Int where
> bar i = 5
> instance FooBar Int Int
>
> main = do
> print $ bar (4::Int)
> print $ foobar (5::Int) (2::Int)
It seems that the "DefaultSignatures" extension is not necessary, the
following version works correctly on my computer (ghc 7.6.2):
-------
{-# LANGUAGE MultiParamTypeClasses #-}
class Bar a where
bar :: a -> Int
class FooBar a b where
foobar :: Bar a => a -> b -> Int
foobar avalue bvalue = bar avalue
instance Bar Int where
bar i = 5
instance FooBar Int Int
main = do
print $ bar (4::Int)
print $ foobar (5::Int) (2::Int)
-------
------------------------------
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