Re: [Haskell-beginners] Re: monad nomad gonad gomad

14 Aug 2010

      For your string-manipulation problem, I don't think you meant to use 
do-notation. The compiler accepted it because sees it as the list monad, 
and as a coincidence of the way you wrote it, it did what you expected.

Try this, using (let ... in ) syntax.
...
mkTxt :: (IConnection conn) => conn -> String -> [String]
mkTxt conn tS =
    let wL          = words (rpNls tS)
        ((f,vL):zz) = gtInx wL ["```"]
        rvL         = reverse vL
    in doIns wL rvL
        where doIns wL []     = wL
              doIns wL (v:vs) =
                  let (f,a:b:ss) = splitAt v wL
                  in (doIns f vs) ++ ["aoeeuu"] ++ ss
(Untested.)

Next readFile has the signature
readFile :: FilePath -> IO String

So the way I think of it is that any function using it has to return a 
result of "IO <something>"

You would have to call your pure function mkTxt from inside a monadic 
computation:

run :: IO ()
run = do
   zzzz <- readFile "zzpubs.txt"
   ...
   -- assuming mkTxt is modified to accept a third arg
   let result = mkTxt conn ts zzzz
   print result

Finally, yes Haskell complains about type-related faults in identifiers you 
don't use... because it is trying to help you find your mistakes. How does 
it know it wasn't a mistake on your part?

This is an advantage over script languages.

Mike

prad wrote:
...
On Fri, 13 Aug 2010 21:39:33 -0700
prad  wrote:
...
"I think It's time for you to get serious with the monads"
that's just what i'm going to do!
i'm asking the question in this thread because i think it has something
to do with monads though i'm not sure. in fact, the problem seems
completely bizarre to me.
i have a function:
mkTxt :: (IConnection conn) => conn -> String -> [String]
mkTxt conn tS = do
    --zzzz <- readFile "zzpubs.htm"
    let wL          = words (rpNls tS)
        ((f,vL):zz) = gtInx wL ["```"]
        rvL         = reverse vL
    doIns wL rvL
        where doIns wL []     = wL
              doIns wL (v:vs) = do
                  let (f,a:b:ss) = splitAt v wL
                  (doIns f vs) ++ ["aoeeuu"] ++ ss
the program compiles and runs fine.
however, if i remove the comment dashes to allow
zzzz <- readFile "zzpubs.htm"
the compiler produces what is to me an incomprehensible rationale for
an error:
====
 gadit.hs:103:4:
    Couldn't match expected type `IO String'
           against inferred type `[String]'
    In a stmt of a 'do' expression: zzzz <- readFile "zzpubs.htm"
    In the expression:
        do { zzzz <- readFile "zzpubs.htm";
             let wL = words (rpNls tS)
                 ((f, vL) : zz) = gtInx wL ...
                 ....;
             doIns wL rvL }
    In the definition of `mkTxt':
        mkTxt conn tS
                = do { zzzz <- readFile "zzpubs.htm";
                       let wL = ...
                           ....;
                       doIns wL rvL }
                where
                    doIns wL [] = wL
                    doIns wL (v : vs)
                            = do { let ...;
                                   .... }
====
i don't do anything with zzzz!!
it merely is the name i'm giving to the monadic computation to read in
a file. in fact, it has nothing to do with the rest of the function
because i don't use it at all.
why is the compiler complaining?

Re: [Haskell-beginners] Re: monad nomad gonad gomad

Michael Mossey