[Haskell-cafe] Fwd: [Haskell-beginners] monad and variable result

[Seph Shewell Brockway]

On Mon, Dec 10, 2018 at 06:06:30PM +0100, Damien Mattei wrote:
> for now i'm here:
> 
> getBD :: Connection -> String -> Float
> getBD conn name = noBDfp
>   where qry_head = "select `N° BD` from sidonie.Coordonnées where Nom =
> ?" :: Query
>         -- bd_rows =
>         --   do
>         --     local_bd_rows <- query conn qry_head (Only (name::String))
>         --     return local_bd_rows
>         bd_rows :: IO [Only Text]
>         bd_rows = query conn qry_head (Only (name::String))
>         noBDtxt :: [Text]
>         noBDtxt = fromOnly (Prelude.head bd_rows)
>         noBDstr :: String
>         noBDstr = Text.unpack noBDtxt :: String
>         noBDfp = read $ noBDstr :: Float

Okay, I think I understand how your code is structured now. The point to
recognize is that bd_rows has type IO [Only Text], which is not the same
type as [Only Text]. Prelude.head has the type [a] -> a, and so it can’t
be used on bd_rows as-is. Fortunately, being a monad, IO has an instance
of Functor for free, and we can go from
   
   head :: [Only Text] -> Only Text

   to

   fmap head :: IO [Only Text] -> IO (Only Text)

which takes an IO operation returning [Only Text] and applies head to
its result, giving an IO operation returning an Only Text.

Can you see how the IO monad follows you through everything that uses
its result? If you rewrite noBDtxt as

   noBDtxt :: IO [Text]
   noBDtxt = fmap (fromOnly . Prelude.head) bd_rows

then noBDstr has to be rewritten in a similar way, and so on through to
the final result, giving the main function a type of
Connection -> String -> IO Float.

A lot of the functions in your where clause can be amalgamated, for
example by combining noBDtxt and noBDstr as

   noBDstr = fmap (Text.unpack . fromOnly . Prelude.head)

Similarly, getBD takes the result of noBDfp and returns it unaltered, so
why not just write
   
   getBD = fmap read noBDstr

?

Let me know if you would like me to explain anything in this message in more
detail.

Regards,

Seph

-- 
Seph Shewell Brockway, BSc MSc (Glas.)