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Today's Topics:
1. [IO String] to IO [String] (Ovidiu D)
2. Quickcheck2 - writing a modifier for test data (Nathan H?sken)
3. Re: [IO String] to IO [String] (Tony Morris)
4. Re: [IO String] to IO [String] (Nathan H?sken)
5. Re: [IO String] to IO [String] (Brent Yorgey)
6. Re: Quickcheck2 - writing a modifier for test data (Brent Yorgey)
7. Re: [IO String] to IO [String] (Ovidiu D)
----------------------------------------------------------------------
Message: 1
Date: Sun, 31 Mar 2013 13:19:41 +0300
From: Ovidiu D <[email protected]>
Subject: [Haskell-beginners] [IO String] to IO [String]
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Message-ID:
<CAKVsE7vhTPDuDfN7fQc7N48ibCUQ=m5tegpc2rkkcx7ijuc...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
I have the function f which reads lines form the stdin and looks like this:
f :: [IO String]
f = getLine : f
What I don't like is the fact that the line processing I'm doing will have
to be in the IO Monad
I would like to make this function to have the signature
f : IO [String]
...such that I can get rid of the IO monad and pass the pure string list to
the processing function.
Can I do this?
Thanks
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Message: 2
Date: Sun, 31 Mar 2013 12:21:48 +0200
From: Nathan H?sken <[email protected]>
Subject: [Haskell-beginners] Quickcheck2 - writing a modifier for test
data
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hey,
Using quickcheck2 I want to generate test data, that is a tuple of
integers where both elements are >=0 and < 8.
So I am trying to write a modifier (in the sense of the modifiers found
in Modifiers.hs):
newtype ValidPos = ValidPos Pos deriving Show
instance Arbitrary ValidPos where
arbitrary = do
x <- elements [0..7]
y <- elements [0..7]
return (ValidPos (x,y))
prop_dummy (ValidPos pos) = fst pos > 0
tests = [ testGroup "Tests" [
testProperty "dummy" prop_dummy
]
]
main = defaultMain tests
doing so gives me this error:
No instance for (QuickCheck-2.4.2:Test.QuickCheck.Arbitrary.Arbitrary
ValidPos)
arising from a use of `testProperty'
Possible fix:
add an instance declaration for
(QuickCheck-2.4.2:Test.QuickCheck.Arbitrary.Arbitrary ValidPos)
In the expression: testProperty "dummy" prop_dummy
In the second argument of `testGroup', namely
`[testProperty "dummy" prop_dummy]'
In the expression:
testGroup "Tests" [testProperty "dummy" prop_dummy]
I declared the instance, why canit not be found?
Thanks!
Nathan
------------------------------
Message: 3
Date: Sun, 31 Mar 2013 20:23:14 +1000
From: Tony Morris <[email protected]>
Subject: Re: [Haskell-beginners] [IO String] to IO [String]
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
You can use sequence which will turn your [IO String] into a IO
[String]. If you want to "map a function" along the way, you can use
mapM (or traverse):
That is:
mapM :: (String -> IO b) -> [IO String] -> IO [b]
For further reading, there is a paper about this function called The
Essence of the Iterator Pattern.
On 31/03/13 20:19, Ovidiu D wrote:
> I have the function f which reads lines form the stdin and looks like
> this:
>
> f :: [IO String]
> f = getLine : f
>
> What I don't like is the fact that the line processing I'm doing will
> have to be in the IO Monad
>
> I would like to make this function to have the signature
> f : IO [String]
> ...such that I can get rid of the IO monad and pass the pure string
> list to the processing function.
>
> Can I do this?
>
> Thanks
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
--
Tony Morris
http://tmorris.net/
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Message: 4
Date: Sun, 31 Mar 2013 12:24:33 +0200
From: Nathan H?sken <[email protected]>
Subject: Re: [Haskell-beginners] [IO String] to IO [String]
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Try
sequence :: Monad m => [m a] -> m [a]
I thinkg
sequence f :: IO [String]
should be what you want.
On 03/31/2013 12:19 PM, Ovidiu D wrote:
> I have the function f which reads lines form the stdin and looks like this:
>
> f :: [IO String]
> f = getLine : f
>
> What I don't like is the fact that the line processing I'm doing will
> have to be in the IO Monad
>
> I would like to make this function to have the signature
> f : IO [String]
> ...such that I can get rid of the IO monad and pass the pure string list
> to the processing function.
>
> Can I do this?
>
> Thanks
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
------------------------------
Message: 5
Date: Sun, 31 Mar 2013 06:29:22 -0400
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] [IO String] to IO [String]
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=iso-8859-1
Unfortunately, with the definition
f = getLine : f
this will not work. 'sequence f' has to do *ALL* the IO before you
can process even the first String in the resulting list. Since it is
infinite, it will just sit there reading lines forever but never
letting you process them.
I think in this case using [IO String] is actually a good solution.
-Brent
On Sun, Mar 31, 2013 at 12:24:33PM +0200, Nathan H?sken wrote:
> Try
>
> sequence :: Monad m => [m a] -> m [a]
>
> I thinkg
>
> sequence f :: IO [String]
>
> should be what you want.
>
> On 03/31/2013 12:19 PM, Ovidiu D wrote:
> >I have the function f which reads lines form the stdin and looks like this:
> >
> >f :: [IO String]
> >f = getLine : f
> >
> >What I don't like is the fact that the line processing I'm doing will
> >have to be in the IO Monad
> >
> >I would like to make this function to have the signature
> >f : IO [String]
> >...such that I can get rid of the IO monad and pass the pure string list
> >to the processing function.
> >
> >Can I do this?
> >
> >Thanks
> >
> >
> >_______________________________________________
> >Beginners mailing list
> >[email protected]
> >http://www.haskell.org/mailman/listinfo/beginners
> >
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
------------------------------
Message: 6
Date: Sun, 31 Mar 2013 06:30:58 -0400
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] Quickcheck2 - writing a modifier for
test data
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=iso-8859-1
On Sun, Mar 31, 2013 at 12:21:48PM +0200, Nathan H?sken wrote:
>
> No instance for (QuickCheck-2.4.2:Test.QuickCheck.Arbitrary.Arbitrary
> ValidPos)
The fact that it actually lists the package and version number in the
error message strongly suggests that the problem is conflicting
versions of the QuickCheck package. Do 'ghc-pkg list QuickCheck' to
see if you have multiple versions installed, and unregister all but
one of them.
-Brent
------------------------------
Message: 7
Date: Sun, 31 Mar 2013 13:48:23 +0300
From: Ovidiu D <[email protected]>
Subject: Re: [Haskell-beginners] [IO String] to IO [String]
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Message-ID:
<CAKVsE7sLTL=hmgfkjuwm5mahftkvf4ebrxmvfyvjmdgsdzw...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Right. I definitely want the lazy behavior.
Thanks
On Sun, Mar 31, 2013 at 1:29 PM, Brent Yorgey <[email protected]>wrote:
> Unfortunately, with the definition
>
> f = getLine : f
>
> this will not work. 'sequence f' has to do *ALL* the IO before you
> can process even the first String in the resulting list. Since it is
> infinite, it will just sit there reading lines forever but never
> letting you process them.
>
> I think in this case using [IO String] is actually a good solution.
>
> -Brent
>
> On Sun, Mar 31, 2013 at 12:24:33PM +0200, Nathan H?sken wrote:
> > Try
> >
> > sequence :: Monad m => [m a] -> m [a]
> >
> > I thinkg
> >
> > sequence f :: IO [String]
> >
> > should be what you want.
> >
> > On 03/31/2013 12:19 PM, Ovidiu D wrote:
> > >I have the function f which reads lines form the stdin and looks like
> this:
> > >
> > >f :: [IO String]
> > >f = getLine : f
> > >
> > >What I don't like is the fact that the line processing I'm doing will
> > >have to be in the IO Monad
> > >
> > >I would like to make this function to have the signature
> > >f : IO [String]
> > >...such that I can get rid of the IO monad and pass the pure string list
> > >to the processing function.
> > >
> > >Can I do this?
> > >
> > >Thanks
> > >
> > >
> > >_______________________________________________
> > >Beginners mailing list
> > >[email protected]
> > >http://www.haskell.org/mailman/listinfo/beginners
> > >
> >
> >
> > _______________________________________________
> > Beginners mailing list
> > [email protected]
> > http://www.haskell.org/mailman/listinfo/beginners
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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