[Haskell-cafe] fmap use

[Damien Mattei]

hello again,

another question with Haskell, again with my database but that is again
related to Monads:

i have this code in main:

 rows <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE
distance > 0.000278"

    let lg = Prelude.length rows

    let tst = if lg > 1 then "some" else "one or zero"

it compiles and i assume it works,

and i have a new (for those that have follow my previous posts) function
like this one:

-- this function will return the list of N°BD from WDS for a given name
-- note: there could be multiple result for a given name in WDS due to
components
getBD_WDS :: Connection -> String -> IO Float
getBD_WDS conn name = trace "Entering getBD_WDS" noBDfp
  where qry_head = "select DNUM from WDS where DISC = ?" :: Query

        bd_rows :: IO [Only Text]
        bd_rows = query conn qry_head (Only (name::String))

        lg = fmap Prelude.length bd_rows

--        tst = if (fmap lg) > 1 then "some" else "one or zero"

        noBDtxt :: IO Text
--        noBDtxt = trace "lg " (fmap (fromOnly . Prelude.head) bd_rows)
        noBDtxt = (fmap (fromOnly . Prelude.head) bd_rows)
--        noBDstr :: String
--        noBDstr = Text.unpack noBDtxt
        noBDstr :: IO String
        noBDstr = fmap Text.unpack noBDtxt
--        noBDfp = read $ noBDstr :: Float
        noBDfp :: IO Float
        noBDfp = trace "Exiting getBD" (fmap read noBDstr)

the problem is that

first i noticed that in a function to make working  the same code than
in Main i had to had almost always fmap before the line,
i admit ,even if i'm teacher in functional programming in Scheme at
University,
i begin in Haskell and i don't always understand what i'm using,
i try to learn it by instinct even if sometimes i read pages about
Functor and Monads i have no time to get in the Mathematics theoretical
knowledge behind Haskell, i try to learn by getting some solutions in
Haskell-cafe and it works, i know now a lot more about Haskell than 2
weeks ago,so....

if someon could explain me a few more about fmap or just give what i
must place in my function instead of
let lg = Prelude.length rows

let tst = if lg > 1 then "some" else "one or zero"

i replace this in the function by:

lg = fmap Prelude.length bd_rows
OK for compilation, but then

tst = if lg > 1 then "some" else "one or zero"
does not compile:

*Main> :load UpdateSidonie
[1 of 1] Compiling Main             ( UpdateSidonie.hs, interpreted )

UpdateSidonie.hs:80:18: error:
    • No instance for (Ord (IO Int)) arising from a use of ‘>’
    • In the expression: lg > 1
      In the expression: if lg > 1 then "some" else "one or zero"
      In an equation for ‘tst’:
          tst = if lg > 1 then "some" else "one or zero"
   |
80 |         tst = if lg > 1 then "some" else "one or zero"
   |                  ^^^^^^

UpdateSidonie.hs:80:23: error:
    • No instance for (Num (IO Int)) arising from the literal ‘1’
    • In the second argument of ‘(>)’, namely ‘1’
      In the expression: lg > 1
      In the expression: if lg > 1 then "some" else "one or zero"
   |
80 |         tst = if lg > 1 then "some" else "one or zero"
   |                       ^
Failed, no modules loaded.


and even the "fmap miraculous function" as i call it no more works:

tst = if (fmap lg) > 1 then "some" else "one or zero"


Prelude> :load UpdateSidonie
[1 of 1] Compiling Main             ( UpdateSidonie.hs, interpreted )

UpdateSidonie.hs:80:24: error:
    • Couldn't match expected type ‘a0 -> b0’ with actual type ‘IO Int’
    • In the first argument of ‘fmap’, namely ‘lg’
      In the first argument of ‘(>)’, namely ‘(fmap lg)’
      In the expression: (fmap lg) > 1
   |
80 |         tst = if (fmap lg) > 1 then "some" else "one or zero"
   |                        ^^
Failed, no modules loaded.

a solution to this proble i think , will help me a lot,
i sticked with problems in haskell i would have already solved in Scheme
or Python, that's really discouraging me of using Haskell, i'm
obstinating myself because i really think it's a great language that
push the programming to high levels of abstractions but for now it's a
bit a nightmare, i must admit it...






-- 
Damien.Mattei at unice.fr, Damien.Mattei at oca.eu, UNS / OCA / CNRS