22 lines
695 B
Haskell
22 lines
695 B
Haskell
module Evaluator where
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import Expression
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import Primitives
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-- Evals (reduces) an expression
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eval :: Expression -> Either String Expression
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eval n@(Number _) = Right n
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eval s@(Symbol _) = Right s
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eval (SExpr es) = apply . SExpr =<< mapM eval es
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-- Applies, in a SExpr, a function to its arguments
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apply :: Expression -> Either String Expression
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apply (SExpr (f:args)) = case f of
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Symbol "+" -> Right $ pAdd args
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Symbol "-" -> Right $ pSub args
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Symbol "*" -> Right $ pMul args
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Symbol "/" -> Right $ pDiv args
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Symbol x -> Left $ show x ++ ": no primitive functions not supported"
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_ -> Left "object not applicable"
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apply _ = Left "object not applicable"
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