<div dir="ltr">Hi Francesco,<div><br></div><div>Thank you for your response, it definitely helped. Below is an updated version of working code. Where I was getting tripped up earlier was in the commented portion below. Also, if you scroll down further, I have a few further questions. Thank you for your time.</div><div><br></div><div><div>module Main where</div><div><br></div><div>import Evaluator</div><div>import System.Console.Haskeline</div><div><br></div><div>main :: IO ()</div><div>main = runInputT defaultSettings loop</div><div> where</div><div> loop :: InputT IO ()</div><div> loop = do</div><div> line <- getInputLine ">> "</div><div> case line of</div><div> Nothing -> return ()</div><div> Just input -> do</div><div> let (result, next) = evaluate input</div><div> if (next == Return)</div><div> then return ()</div><div> else (emit result >> loop) <font color="#0000ff">-- I had wanted the result to be emitted only on Continue, not Return. This works, but is it a good way?</font></div><div><br></div><div>emit :: Maybe String -> InputT IO ()</div><div>emit Nothing = return ()</div><div>emit (Just value) = outputStrLn $ "Executing: " ++ value</div></div><div><br></div><div><div>module Evaluator where</div><div><br></div><div>data Next = Continue | Return</div><div> deriving (Eq, Ord)</div><div><br></div><div>evaluate :: String -> (Maybe String, Next) <font color="#0000ff">-- Now updated to make the first part of the tuple a Maybe String instead of String</font></div><div>evaluate "quit" = (Nothing, Return)</div><div>evaluate value = (Just value, Continue)</div></div><div><br></div><div><span style="line-height:1.5">** QUESTIONS **</span><br></div><div><br></div><div>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.</div><div>2. You mentioned outputStrLn has a type `<span style="color:rgb(33,33,33);font-family:"helvetica neue",helvetica,arial,sans-serif;line-height:1.5">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?</span></div><div><span style="color:rgb(33,33,33);font-family:"helvetica neue",helvetica,arial,sans-serif;line-height:1.5">3. Are there better/simpler/more idiomatic ways of structuring my program?</span></div></div><br><div class="gmail_quote"><div dir="ltr">On Sun, Jul 31, 2016 at 12:01 PM Francesco Ariis <<a href="mailto:fa-ml@ariis.it">fa-ml@ariis.it</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">On Sun, Jul 31, 2016 at 06:44:14PM +0000, Ramnath R Iyer wrote:<br>
> The program below is what I have working right now (very trivial). I want<br>
> to modify this program, so that the `evaluate input` returns not a String,<br>
> but a type X that includes information on whether to "continue" and show a<br>
> prompt after rendering the result, or exit altogether. So the specific<br>
> questions I have here are:<br>
><br>
> 1. What would be a sensible type signature for X?<br>
<br>
Hello Ramnath,<br>
<br>
as now evaluate *has to* return a String because you pass its output as<br>
an argument to outputStrLn (which has a ~ `String -> InputT IO ()`<br>
signature itself).<br>
<br>
A quick hack is for `evaluate` to return<br>
<br>
evaluate :: String -> (String, Bool)<br>
<br>
where the Bool stands for 'should I exit or not' (using a datatype<br>
and/or a type synonym would be better and more clear).<br>
You can then write<br>
<br>
let (s, b) = evaluate input<br>
outputStrLn $ "Something " ++ s<br>
if b then loop<br>
else exit<br>
<br>
Your code has a very distinct imperative flavour (there are ways<br>
to get rid of that case/if cascade), but my opinion<br>
is: first make it work, then make it pretty.<br>
<br>
Does that help?<br>
-F<br>
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</blockquote></div>