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Today's Topics:
1. foldM with tuple and maybe (Miro Karpis)
2. Re: foldM with tuple and maybe (David McBride)
----------------------------------------------------------------------
Message: 1
Date: Tue, 31 Dec 2013 00:42:26 +0100
From: Miro Karpis <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] foldM with tuple and maybe
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<cajnnbxhffjm+oedbtoq0gxb7oxzfr+gpupfng7reteupgbp...@mail.gmail.com>
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Hi, please can you help me with following?
I have a following function:
f1 :: (String, String) -> IO (Maybe String)
which I would like to apply to a list of tuples.I have defined the function
like this, but that gives me error (below):
f2 :: [(String, String)] -> IO (Maybe String)
f2 x = foldM f1 0 x
error:
Couldn't match expected type `IO (Maybe String)' with actual type `[b0] ->
m0 (String, String)' In the return type of a call of `foldM'
thanks,
m.
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Message: 2
Date: Mon, 30 Dec 2013 19:01:44 -0500
From: David McBride <[email protected]>
To: Miro Karpis <[email protected]>, The Haskell-Beginners
Mailing List - Discussion of primarily beginner-level topics related
to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] foldM with tuple and maybe
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<CAN+Tr41SFEmcbwRSeatPuNyvZ0k-Z=bto_6mnru8khtj_-o...@mail.gmail.com>
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You have a foldM function which takes a " a -> b -> m a " as a first
argument. But you are passing it f1 which has a type (a, b) -> m a.
Unfortunately I'm not sure what you were trying to do. The function
you have just doesn't fit. :(
Maybe you intended to have f1 :: String -> (String, String) -> IO
String, and then go
:t foldM f1 "" x
foldM f1 "" [("a","b"),("c","d")] :: IO String
?
On Mon, Dec 30, 2013 at 6:42 PM, Miro Karpis <[email protected]> wrote:
> Hi, please can you help me with following?
>
> I have a following function:
> f1 :: (String, String) -> IO (Maybe String)
>
> which I would like to apply to a list of tuples.I have defined the function
> like this, but that gives me error (below):
>
> f2 :: [(String, String)] -> IO (Maybe String)
> f2 x = foldM f1 0 x
>
>
> error:
> Couldn't match expected type `IO (Maybe String)' with actual type `[b0] ->
> m0 (String, String)' In the return type of a call of `foldM'
>
> thanks,
> m.
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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