[Haskell-beginners] Maybe Return?

[Francesco Ariis]

On Sun, Jul 31, 2016 at 06:44:14PM +0000, Ramnath R Iyer wrote:
> The program below is what I have working right now (very trivial). I want
> to modify this program, so that the `evaluate input` returns not a String,
> but a type X that includes information on whether to "continue" and show a
> prompt after rendering the result, or exit altogether. So the specific
> questions I have here are:
> 
> 1. What would be a sensible type signature for X?

Hello Ramnath,

    as now evaluate *has to* return a String because you pass its output as
an argument to outputStrLn (which has a ~ `String -> InputT IO ()`
signature itself).

A quick hack is for `evaluate` to return

    evaluate :: String -> (String, Bool)

where the Bool stands for 'should I exit or not' (using a datatype
and/or a type synonym would be better and more clear).
You can then write

    let (s, b) = evaluate input
    outputStrLn $ "Something " ++ s
    if b then loop
         else exit

Your code has a very distinct imperative flavour (there are ways
to get rid of that case/if cascade), but my opinion
is: first make it work, then make it pretty.

Does that help?
-F