[Haskell-beginners] Maybe Return?

[Francesco Ariis]

On Sun, Jul 31, 2016 at 09:17:09PM +0000, Ramnath R Iyer wrote:
> Below is an updated version of working code.
> [...]

Well done! Now, to the questions:

> 1. If I wanted to write the logic encapsulated in `emit' directly within
> main, how would I do that? In my example, I was forced to extract it out as
> a separate method specifically to leverage pattern matching.

Pattern matching can be done via a `case` statement as well (and
you provide an example of this just a few lines above the `emit`
call).
Personally, I much a prefer separate function as it is clearer (if you
don't want to clutter top level use a `where` statement).

> 2. You mentioned outputStrLn has a type `String -> InputT IO ()'. How do
> you logically come to this conclusion? Is it because outputStrLn was taking
> a single String argument and had to return the same type returned by loop
> declared previously?

Correct, if we ask ghci about `ouputStrLn` it will reply:

    λ> :t outputStrLn
    outputStrLn :: Control.Monad.IO.Class.MonadIO m => String -> InputT m ()

but in this case you have written the signature to `loop` (it's always a
good habit), so it's clear our function can only have this concrete
data type.

> 3. Are there better/simpler/more idiomatic ways of structuring my program?

Yes there are: your program checks/pattern matches at every step and for
each of those you have a case/if; it could get pretty messy if you were
to add a few more actions. There is a nice way to handle these kind of
situations, it involves monadic code.
But before using monads you need to digest them! So keep following
your course (if you are not following any, I like these two, which
are gratis [1] [2]) and come back to this .hs after you tamed the
mighty monad to refactor it!

[1] http://learnyouahaskell.com/
[2] https://www.seas.upenn.edu/~cis194/spring13/lectures.html