我有一個簡短的問題。哈斯克爾扔我的57 - Undefined variable "f" error
,我不知道爲什麼。如果你能看看這個,我會很感激。未定義的變量,Haskell
代碼:
eval :: Expr -> Environment -> Float
eval expr env = eval' expr
where
eval' :: Expr-> Float
eval' (Num num) = num
eval' (App app exprs) = foldl1 (f) (map eval' exprs) -- **Line 57**
eval' (Id id) = 5
where
f = getFunctionForApp app -- **f is here**
getFunctionForApp :: String -> (Float->Float->Float)
getFunctionForApp "+" = (+)
getFunctionForApp "-" = (-)
getFunctionForApp "*" = (*)
getFunctionForApp "/" = (/)
getIdVal :: String -> Environment -> Float
getIdVal id ((curId, val):envrs)
|curId == id = val
| otherwise = getIdVal id envrs
類型定義:
data Expr = Num Float | Id String | App String [ Expr ]
deriving (Eq, Ord, Show)
type Environment = [ (String, Float) ]
我誠實地不知道答案,但我認爲在發表聲明後必須提到where塊。換句話說,你是否嘗試過移動整個阻塞一條線? – Ramy