[Haskell-beginners] Combining IO and Either function to "EitherT e IO a"

[Mateusz Kowalczyk]

On 05/03/14 14:31, Mateusz Kowalczyk wrote:
> On 05/03/14 13:53, Nathan Hüsken wrote:
>> Hey,
>>
>> I have a function
>>
>> |func1 :: IO String
>> |
>>
>> and another:
>>
>> |func2 :: String -> Either String String
>> |
>>
>> and I want to combine them, giving the output of the first as the input
>> as the second.
>>
>> |func3 :: IO (Either String String)
>> func3 = do
>>   tmp <- func1
>>   return (func2 tmp)
>> |
>>
> 
> This is just ‘fmap func2 func1’ or using the operator, ‘func2 <$> func1’.
> 
>> Ok, possible. But I rather would like a result of type "EitherT String
>> IO String".
>> So how can I combine these function in a smart way, to get the needed
>> result?
> 
> After using ‘func2 <$> func1’ we have ‘IO (Either String String)’ as you
> point out. The ‘EitherT’ constructor has type ‘m (Either e a)’. 

Oops, that meant to say that it takes a sole argument of that type.

> Here if
> m = IO, e = String, a = String so we have exactly what we need:
> 
>>> :t EitherT $ func2 <$> func1
>> EitherT $ func2 <$> func1 :: EitherT String IO String
> 
>>
>> Thanks!
>> Nathan
>>
>>
>>
>>
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> 
> 


-- 
Mateusz K.