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chapter3/calc/Eval.hs: avoid recomputation of normal form in eval

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This commit is contained in:
Christian Sievers 2015-01-30 20:23:57 +01:00
parent 11709255c3
commit 7d2ecc4b9a
1 changed files with 3 additions and 3 deletions
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6
chapter3/calc/Eval.hs
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@ -34,6 +34,6 @@ nf :: Expr -> Expr
nf x = fromMaybe x (nf <$> eval' x)
eval :: Expr -> Maybe Expr
eval t = case isVal (nf t) of
True -> Just (nf t)
False -> Nothing -- term is "stuck"
eval t = case nf t of
nft | isVal nft -> Just nft
| otherwise -> Nothing -- term is "stuck"