[Haskell-beginners] Re: monad nomad gonad gomad

[prad]

On Sun, 15 Aug 2010 21:11:52 +0200
Ertugrul Soeylemez <es at ertes.de> wrote:

> Just remember this:  You have an IO computation?  You want to refer to
> its result?  Fine, just use '<-'.  That's it.
>
that's what i've been doing the past two days and it works ... except
in one case.

for example, i have:

========
    flData <- readFile key
    let (sec:tle:kwd:dsc:url:typ:dat:txt) = lines flData
========

so flData is the computation which reads the contents of a file and i
can take that and do a lines on it. no problem.

however, i also have:

========
subs (l:ls)
    | ls==[]    = l : []
    | otherwise = 
        case l of
            "```"   -> do
                let (code:z)   = ls
                    str        = gt code    -- works
                --str <- readFile "B"       -- doesn't work
                subs (str:z)
            _       -> l : subs ls


gt s =
   case s of
        "B"    -> "BBBB"
        "C"    -> "CCCC"
========

now str = gt code is fine because it's not monadic. however, if i try
to get the data from a file or a db, it gives the error

    Couldn't match expected type `[[Char]]'
           against inferred type `IO b'

because i'm trying to get an IO monad into a [String] from what i
understand.

so i'm trying different things here, but haven't found a way yet.


> In fact, I recommend not to think more, but to think less.
>
ok! i'll see what i can do!
as you rightly keep telling me, i'm being way too complicated. :D

i'm enjoying your extensive tutorial:
http://ertes.de/articles/monads.html

though it's going slowly, but things are coming together little by
little.

i was also pointed to this:
http://codingcactus.wordpress.com/2010/08/16/writemonad1/

and i just saw mike vanier's post here:
very detailed monad tutorials (a series of articles)
http://mvanier.livejournal.com/3917.html

so with all this help i'm sure to 'get it' eventually!

> At some point it will make click and suddenly everything gets clear.
> Everybody has this experience with monads (unless they give up).
>
no worries about that!
i do see the light in the long tunnel - now it's just a matter of
getting to it.

-- 
In friendship,
prad

                                      ... with you on your journey
Towards Freedom
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