[Haskell-beginners] GHC not buying what I am offering this afternoon

[Sumit Sahrawat, Maths & Computing, IIT (BHU)]

Just like sequence, there is also a function sequence_ that you might want
to use.

    sequence_ :: Monad m => [m a] -> m ()    -- for m = IO, sequence_ ::
[IO a] -> IO ()
    sequence_ lst = do
      sequence lst
      return ()

    -- also, mapM and mapM_ are useful too
    mapM f  = sequence . map f      -- retains results
    mapM_ f = sequence_ . map f     -- ignores results

    -- Some use cases
    ghci> sequence [print 2, print 3]
      2
      3
      [(),()]      -- The results, each print is of type IO (), i.e does IO
and produces ()
    ghci> mapM print [2,3] -- same as above
    ghci> mapM_ print [2,3] -- same as above, but without results (or
result of type unit, i.e ())


On 10 March 2015 at 02:09, Joel Williamson <joel.s.williamson at gmail.com>
wrote:

> sequence will get the types to match up, but a more elegant solution
> would be to get every line into a single string, then print that.
>
> putStrLn $ unlines $ map show newList
>
> I agree that getting the types to line up can be a nuisance, and with
> such small programs it doesn't bring much of an advantage. Ultimately,
> if you want side-effects to be reflected in the type system, there
> will be times you have to do a bit of extra work to satisfy the type
> checker. Learning to write well-typed code is much easier in this
> context, where everything has a fairly concrete type and there is lots
> of documentation, than having to learn it later when you have weird
> types coming from 3 different libraries and are facing a problem no
> one else has had. I would recommend getting very comfortable with GHCi
> and Hoogle.
>
> If you haven't already, add a hoogle prompt to GHCi by pasting something
> like
> :def hoogle \str -> return $ ":! hoogle --count=15 \"" ++ str ++ "\""
> in your ghci.conf. This will allow you to easily search for functions
> of a given type. Typing
> :hoogle [IO a] -> IO [a]
> returned all the information needed to answer your question.
>
>
> On Mon, Mar 9, 2015 at 4:25 PM, Geoffrey Bays <charioteer7 at gmail.com>
> wrote:
> > Thanks, Joel.
> >
> > Putting the type IO [()] in the main declaration and this as the final
> line
> > of the main function does do the trick:
> >
> > sequence [putStrLn $ show s | s <- newList]
> >
> > But this is the kind of thing that makes Haskell types difficult for
> > beginners to work with...
> >
> > Geoffrey
> >
> > On Mon, Mar 9, 2015 at 4:15 PM, Joel Williamson
> > <joel.s.williamson at gmail.com> wrote:
> >>
> >> main must have type IO a. Hoogle tells me that to convert [IO a] -> IO
> >> [a], you should use the function sequence. Try applying that to your
> final
> >> line.
> >>
> >>
> >> On Mon, 9 Mar 2015 16:07 Geoffrey Bays <charioteer7 at gmail.com> wrote:
> >>>
> >>> My main function looks like this:
> >>>
> >>> main :: [IO()]
> >>> main = do
> >>>     let stud1 = Student {name = "Geoff", average = -99.0, grades =
> >>> [66,77,88]}
> >>>     let stud2 = Student {name = "Doug", average = -99.0, grades =
> >>> [77,88,99]}
> >>>     let stud3 = Student {name = "Ron", average = -99.0, grades =
> >>> [55,66,77]}
> >>>     let studList = [stud1,stud2]
> >>>     let newList = calcAvg studList
> >>>     [putStrLn $ show s | s <- newList]
> >>>     --putStrLn $ show (newList !! 0)
> >>>     --putStrLn $ show (newList !! 1)
> >>>
> >>> With this final line, putStrLn $ show (newList !! 0), the type IO () in
> >>> the function declaration compiles fine.
> >>> But with [putStrLn $ show s | s <- newList] as the final line, [IO ()]
> in
> >>> the function declaration will not compile, I get this error:
> >>>
> >>>     Couldn't match expected type `IO t0' with actual type `[IO ()]'
> >>>
> >>> What does the declared type need to be for a final line of:
> >>> [putStrLn $ show s | s <- newList]  ???
> >>>
> >>> Thanks,
> >>>
> >>> Geoffrey
> >>>
> >>> _______________________________________________
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> >>
> >>
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> >
> >
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-- 
Regards

Sumit Sahrawat
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