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POData types and non exhaustive patterns
 

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  1. POData types and non exhaustive patterns
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    <p>If have a data type </p> <pre><code>data Arith = Con Int | Add Arith Arith | Sub Arith Arith | Mul Arith Arith | Div Arith Arith </code></pre> <p>an instance: </p> <pre><code>instance Show Arith where show (Con i) = show i show (Add e1 e2) = "(" ++ show e1 ++ " + " ++ show e2 ++ ")" show (Sub e1 e2) = "(" ++ show e1 ++ " - " ++ show e2 ++ ")" show (Mul e1 e2) = "(" ++ show e1 ++ " * " ++ show e2 ++ ")" show (Div e1 e2) = "(" ++ show e1 ++ " / " ++ show e2 ++ ")" </code></pre> <p>and a function evaluate which is expected behave like:</p> <pre><code>ghci&gt; eval (Mul (Con 5) (Div (Con 6) (Con 2) )) 15 </code></pre> <p>this is what I've come up with that's not working when I try anything more complex then the basic case. </p> <pre><code>eval :: ArithExp -&gt; Int eval (Con e1) = e1 eval (Add (Con e1) (Con e2)) = e1 + e2 eval (Sub (Con e1) (Con e2)) = e1 - e2 eval (Div (Con e1) (Con e2)) = e1 `div` e2 eval (Mul (Con e1) (Con e2)) = e1 * e2 </code></pre> <p>When I try anything that isn't in this case, of course I get an error about exhaustive patterns. </p> <p>So my question is, how do I catch all of the patterns without doing it the naive way?</p> <p>I did notice that Add is defined as Add Arith Arith, Sub Arith Arith so how would I take advantage of that in my eval function? I tried doing:</p> <pre><code>eval (Add Arith Arith) = Arith + Arith </code></pre> <p>But that didn't work either, (not in scope error). </p> <p>You don't need to give me a full working code example or anything, I'm just trying to get pointed in the right direction. Still new at haskell. </p> <p>Thanks. </p>
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