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Today's Topics:
1. Good way of combining functions of type IO (Maybe a)
(Nathan H?sken)
2. Re: Good way of combining functions of type IO (Maybe a)
(David McBride)
3. list problem (Roelof Wobben)
4. list problem (Benjamin Edwards)
5. Re: Good way of combining functions of type IO (Maybe a)
(Chris Schneider)
6. Re: list problem (Roelof Wobben)
----------------------------------------------------------------------
Message: 1
Date: Mon, 03 Mar 2014 16:10:05 +0100
From: Nathan H?sken <[email protected]>
To: Haskell Beginners Mailinglist <[email protected]>
Subject: [Haskell-beginners] Good way of combining functions of type
IO (Maybe a)
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hey,
I want to write a function, which is basically a concatenation of
functions of type "IO (Maybe a)".
When they all where of type "Maybe a", no Problem I would simple use the
Maybe monad.
|func :: Maybe c
func = do
a <- f1
b <- d2 a
...
|
but now they are of type "IO (Maybe a)". Is there some way of combing
these in a similar smart way?
Thanks!
Nathan
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Message: 2
Date: Mon, 3 Mar 2014 10:21:03 -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] Good way of combining functions of
type IO (Maybe a)
Message-ID:
<CAN+Tr404r+XWZkmJjfBmwQSOnbdi8sMgwytL7w=fubbkcd0...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
cabal install maybet
import Control.Monad.Maybe
f1 :: IO (Maybe Int)
f1 = return . Just $ 1
d2 :: Int -> IO (Maybe String)
d2 = return . Just . show
blah :: IO (Maybe (Int, String))
blah = do
runMaybeT $ do
a <- MaybeT f1
b <- MaybeT $ d2 a
return (a,b)
Or slightly rewritten:
f1 :: MaybeT IO Int
f1 = return 1
-- f1 = fail "why oh why?!?"
d2 :: Int -> MaybeT IO String
d2 = return . show
blah = do
runMaybeT $ do
a <- f1
b <- d2 a
return (a,b)
On Mon, Mar 3, 2014 at 10:10 AM, Nathan H?sken <[email protected]>wrote:
> Hey,
>
> I want to write a function, which is basically a concatenation of
> functions of type "IO (Maybe a)".
> When they all where of type "Maybe a", no Problem I would simple use the
> Maybe monad.
>
> func :: Maybe cfunc = do
> a <- f1
> b <- d2 a
> ...
>
> but now they are of type "IO (Maybe a)". Is there some way of combing
> these in a similar smart way?
>
> Thanks!
> Nathan
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
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Message: 3
Date: Mon, 03 Mar 2014 17:55:34 +0100
From: Roelof Wobben <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] list problem
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hello,
I have this :
cons8 n l = n : l
x = cons8 8 [1,2,3]
main = print x
This will be the output: [8.1.2.3]
Now the problem is how I can take care that the output will be [1,2,3,4,8]
I have to change the function for it.
I tried switching n and L in the function but that will not work.
Or does I have to do x = cons [1,2,3] 8 and make the function like this
cons8 l n = l : n
Roelof
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------------------------------
Message: 4
Date: Mon, 03 Mar 2014 16:59:16 +0000
From: Benjamin Edwards <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] list problem
Message-ID:
<can6k4nhxttg-4tv+0xa2qcsf2_bsgcpj+ld8gm7fwpx4ve3...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
The problem is that you don't seem to understand how singly linked lists
work. I would recommend studying this first, then understanding how to
append a value to the end of the list in haskell will probably make a lot
more sense.
>
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Message: 5
Date: Mon, 3 Mar 2014 10:00:26 -0700
From: Chris Schneider <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Good way of combining functions of
type IO (Maybe a)
Message-ID:
<caghc4ugtujcn35jwnhhl+uno5cqekakln7ojsjdcufqvwce...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
I'm a beginner as well, and I explored how to use the maybeT transformer
and wrote it up on my blog across a few posts. Mostly they were just for my
own learning purposes, but maybe you can find something in them too?
http://watchchrislearn.com/blog/2013/11/28/playing-with-the-either-monad/
http://watchchrislearn.com/blog/2013/11/30/using-the-either-monad-inside-another-monad/
http://watchchrislearn.com/blog/2013/11/30/eithert-inside-of-io/
http://watchchrislearn.com/blog/2013/12/01/working-entirely-in-eithert/
On Mon, Mar 3, 2014 at 8:21 AM, David McBride <[email protected]> wrote:
> cabal install maybet
>
> import Control.Monad.Maybe
>
> f1 :: IO (Maybe Int)
> f1 = return . Just $ 1
>
> d2 :: Int -> IO (Maybe String)
> d2 = return . Just . show
>
> blah :: IO (Maybe (Int, String))
> blah = do
> runMaybeT $ do
> a <- MaybeT f1
> b <- MaybeT $ d2 a
> return (a,b)
>
> Or slightly rewritten:
>
> f1 :: MaybeT IO Int
> f1 = return 1
> -- f1 = fail "why oh why?!?"
>
> d2 :: Int -> MaybeT IO String
> d2 = return . show
>
> blah = do
> runMaybeT $ do
>
> a <- f1
> b <- d2 a
> return (a,b)
>
>
>
> On Mon, Mar 3, 2014 at 10:10 AM, Nathan H?sken
> <[email protected]>wrote:
>
>> Hey,
>>
>> I want to write a function, which is basically a concatenation of
>> functions of type "IO (Maybe a)".
>> When they all where of type "Maybe a", no Problem I would simple use the
>> Maybe monad.
>>
>> func :: Maybe cfunc = do
>> a <- f1
>> b <- d2 a
>> ...
>>
>> but now they are of type "IO (Maybe a)". Is there some way of combing
>> these in a similar smart way?
>>
>> Thanks!
>> Nathan
>>
>> _______________________________________________
>> 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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Message: 6
Date: Mon, 03 Mar 2014 18:04:01 +0100
From: Roelof Wobben <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] list problem
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
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