[Haskell-beginners] Can't cast an IO String to s Astring

[Rein Henrichs]

Or rather, you can just say do { return something } or do { return $
something } of "something" is a more complicated expression.

On Fri, Oct 23, 2015 at 10:16 AM Rein Henrichs <rein.henrichs at gmail.com>
wrote:

> You can't transform an IO String into a String. This is one of the
> purposes of monads: to make it possible to work with things like IO, where
> it is impossible to turn an IO a into an a. The result of performing an IO
> action can be *bound* using (>>=) or <- in do notation so that the String
> is available for the rest of the computation:
>
> do
>   f <- readFile "ctrl.txt"
>   Right r <- parseCtrl f
>   cherchectrl r "A01"
>
> Note that there's no need to wrap identifiers in (): (r) is the same as r,
> (rez) is the same as rez. And there's no need to say do { let r =
> something; return r }. You can just say do { something }.
>
> On Fri, Oct 23, 2015 at 8:32 AM chanti houda <houdachanti at yahoo.fr> wrote:
>
>> Hello, I'm writing a Haskell code which consists to read a text file,
>> parse it and thansform the parsing result on a specific language.
>> I have a function *affiche *which takes a data type Ctrl and returns a
>> String. This is the transformation funtion.
>> I have also anothe function parsctrl, which parse the contents of a text
>> file ("ctrl.txt") and after looks for a specific value ("A01") in the parse
>> result (function *c**herchectrl*).
>> I need to use the result of the *parsectrl *function in another function
>> *fc*.
>> The code is composed of three functions
>>
>>
>>
>> parsectrl = do
>>  f <- readFile "ctrl.txt"
>>  let Right r = parse  parseCtrl  " " f
>>  let rez =cherchectrl ( r) "A01"
>>  return (rez)
>>
>> fc[]  =[]
>> fc((door,[(all,v1),(alt,v2),(lint,v3),(w,v4),(r,v5),(loc,v6),(
>> etat,v7),(ruin,v8)]):ls ) = ("&OUV ID='"++door ++"', ALLEGE="++show((moi
>> v1)/1000)++", LINTEAU="++show((moi v3)/1000)++", LARGEUR="++show((moi
>> v4)/1000)++", COEF=0.7, ALT="++show((moi v2)/1000)++", LOCIDS='"++v6++"',
>> CTRLID='"++ v7++"', CTRLID_RUIN='"++ v8++" /" ++"\n" ++"&CTRL
>> ID='"++v7++"', " ++ "ON_INI=.FALSE., DURATION=0 / \n"++"&CTRL
>> ID='"++v8++"', LOCID='"++  ((parses("'"++v6++"'"))!!0) ++
>> *affiche(parsectrl)*++ " / \n\n"  )++fc(ls)
>>
>> The parsectrl returns an IO String, but the function affiche needs a
>> String as input and even when I tried to adapt the affiche function to take
>> an  IO String -> String I can't.
>> the result of fc must be a String too.
>>
>> The IO String comes from the parsectrl function.
>> Can you help me to solve this problem: how can I transform an IO String
>> to a String.
>>
>> Thank you by advance.
>>
>>
>>
>>
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>>
>
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