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POcreating a dialogue in command line in haskell, where output depends on previous processed input
 

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  1. POcreating a dialogue in command line in haskell, where output depends on previous processed input
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    <p>I am finishing a program that translates a higher-order language to another and I am having trouble dealing with the interface I am trying to implement. </p> <p>The idea is that it asks the user for a number to be chosen, then checks whether it is a number, if it is, it follows with a line asking to write the given term to be translated. </p> <p>If the number is 4, it has an additional line asking for the term to be substituted, thus the translating function takes 2 args instead of 1. Therefore I'd like to have 3 lines of input for the user to complete depending on the previous answer, however my code asks for a number and then prints the next question and returns directly to the beginning, printing the first lines again.</p> <p>I have noticed that it does not test the validity of the Char taken from the input*<em>(isDigit num)</em>* and always chooses the path of <em>if(ord num /= 4)</em> equals to True, even when I key the number 4!</p> <p>I have tried several paths to sort this out but I am at a lost. I am more than happy to change my interface code completely if you know of any better approaches, because I cannot think of any other.</p> <p>I am quite amateur at Haskell by the way.</p> <pre><code>main :: IO () main = do putStrLn "Choose a definition to be implemented:\n 1 - Def 1 dealing with a Generalised class of CRS terms without permutations,\n 2 - Def 2 which is an extension of Def 1 with permutations,\n 3 - Def 4 which takes a closed nominal term-in-context, returning a closed CRS (meta)term,\n 4 - Def 6 which extends on Def 4 by adding a ground nominal substitution to the arguments.\n Definition number: " num &lt;- getChar unless (isDigit num) (return ()) def num main def :: Char -&gt; IO String def num = do putStrLn "write a nominal term-in-context: " hFlush stdout str &lt;- getLine case num of '1' -&gt; (return . readExp1) str '2' -&gt; (return . readExp2) str '3' -&gt; (return . readExp4) str '4' -&gt; do putStrLn "write a nominal term and the variable it substitutes:" sub &lt;- getLine let term = readExp6 str (readSub sub) return term {- reads and ouputs type TrmCxt ((atm,Var),Trm) -} readExp1 :: String -&gt; String readExp1 s = case (inputL parseT s) of Left err -&gt; "error: " ++ err Right (fc , t) -&gt; show $ translate1 [] fc t </code></pre> <p>I have added to the snippet the function <em>readExp1</em> which parses the input and calls the translating function for you to understand what it does and what it returns. The others are equivalent in their return type.</p>
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