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POHaskell recursive problem, tiny parser. A few things
 

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  1. POHaskell recursive problem, tiny parser. A few things
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    <pre><code>data Expr = Var Char | Tall Int | Sum Expr Expr | Mult Expr Expr | Neg Expr | Let Expr Expr Expr deriving(Eq, Show) </code></pre> <p>That is the datatype for <code>Expr</code>, I have a few questions. I'm suppose to parse expressions like <code>*(Expr,Expr)</code> as shown in the datatype definition. However I do have some problems with "creating" a valid <code>Expr</code>. I use pattern matching for recognizing the different things Expr can be. Some more code: </p> <pre><code>parseExpr :: String -&gt; (Expr, String) parseExpr ('*':'(':x:',':y:')':s) = (Mult (parseExpr [x] parseExpr [y]),s) </code></pre> <p>This is not working, obviously. The return type of <code>parseExpr</code> is to return the rest of the expression that is to be parsed an a portion of the parsed code as an <code>Expr</code>. The right side of this code is the problem. I can't make a valid <code>Expr</code>. The function is suppose to call it self recursively until the problem is solved. </p> <p>ANOTHER problem is that I don't know how to do the pattern matching against <code>Var</code> and <code>Tall</code>. How can I check that <code>Var</code> is an uppercase character between A-Z and that <code>Tall</code> is 0-9 and return it as a valid <code>Expr</code>? </p> <p>Generally I can just look at a few parts of the string to understand what part of <code>Expr</code> I'm dealing with. </p> <pre><code>Input like: parseProg "let X be 9 in *(X , 2)" Would spit out: Let (Var 'X') (Tall 9) (Mult (Var 'X') (Tall 2)) </code></pre>
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    1. COInput like: parseProg "let X be 9 in *(X , 2)" Would spit out: Let (Var 'X') (Tall 9) (Mult (Var 'X') (Tall 2))
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    2. COJust a heads up, SO has code tags. I edited your post to include them. And what you just added in your comment should probably be added into your original post via the edit button.
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    3. COAgreed, thank you.
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