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Created February 20, 2014 11:19
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The non-env using patterns of eval
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trivialEval.hs
eval :: Expr -> EvalSt Int
eval (N n) = return n
eval (Add e1 e2) = do
v1 <- eval e1
v2 <- eval e2
return (v1 + v2)
eval (Sub e1 e2) = do
v1 <- eval e1
v2 <- eval e2
return (v1 - v2)
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