[Haskell-beginners] foldM with tuple and maybe

[Dominik Bollmann]

Or maybe what you really want is mapM instead of foldM? In this case,
your function f2 will look like this:

f2 :: [(String, String)] -> IO [Maybe String]
f2 xs = mapM f1 xs

And this will first map (=apply) your function f1 over all elements in
the given input list xs and then it will sequence this list, thus
creating a resulting list of type IO [Maybe String]

David McBride <toad3k at gmail.com> writes:

> 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 <miroslav.karpis at gmail.com> 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.
>>
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