[Haskell-cafe] Confusing `bind` and (>>=)

[jpaugh at gmx.com]

On October 10, 2017 6:43:41 AM CDT, Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com> wrote:
>On 10 October 2017 at 22:34, mirone <saul-mirone at qq.com> wrote:
>> Hi there,
>>   I'm learning haskell for weeks and I'm trying to write a parser
>using
>> haskell.
>>   First I create a datatype:
>>     newtype Parser a = P (String -> [(a, String)])
>>   I create a function called `bind` :
>>     bind :: Parser a -> ( a -> Parser b) -> Parser b
>>     bind f g = P $ \s -> concatMap (\(a, s') -> parse (g a) s') $
>parse f s
>>   and then:
>>     instance Monad Parser where
>>       (>>=)  = bind
>>   that's looks cool, than I write a function
>>     liftToken :: (Char -> [a]) -> Parser a
>>     liftToken f = P g where
>>       g [] = []
>>       g (c:cs) = f c >>= (\x -> return (x, cs))
>
>Here, f c :: [a], so you're using the [] instance of Monad, hence
>(>>=) :: [a] -> (a -> [b]) -> [b]

Yep, and >>= for [a] will often have different semantics then whatever monad you had in mind. You can inject the list into the parser monad by using return.

    return (f c) `bind` (\x -> return (x, cs))

>
>>   It works well, then I change this function to
>>     liftToken :: (Char -> [a]) -> Parser a
>>     liftToken f = P g where
>>       g [] = []
>>       g (c:cs) = f c `bind` (\x -> return (x, cs))
>>   GHC throw errors:
>>     regular.hs|153 col 14 error| • Couldn't match expected type
>‘Parser t0’
>> with actual type ‘[a]’ • In the first argument of ‘bind’, namely ‘f
>c’ In
>> the expression: f c `bind` (\ x -> return (x, cs)) In an equation for
>‘g’: g
>> (c : cs) = f c `bind` (\ x -> return (x, cs)) • Relevant bindings
>include f
>> :: Char -> [a] (bound at regular.hs:151:11) liftToken :: (Char ->
>[a]) ->
>> Parser a (bound at regular.hs:151:1)
>>
>>   That's really confusing, why I can use (>>=) but I can't use
>`bind`? Is
>> there any difference between them?
>>
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>
>
>
>-- 
>Ivan Lazar Miljenovic
>Ivan.Miljenovic at gmail.com
>http://IvanMiljenovic.wordpress.com
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