[英]Haskell use instanced Read in main with Type
如何在我的main中使用實例讀取?
現在我有以下內容:
data Term = Monom (Float,Int)
| Addition (Term,Term)
| Subtraction (Term,Term)
| Multiplication (Term,Term)
| Division (Term,Term)
deriving(Show)
instance Read Term where
readsPrec _ inp = let[(a,b)] = lex inp in
case a of
"x" -> readsPrec 0 b
"^" -> [(Monom (1.0,(read b::Int)), "")]
c -> let[(d, "")] = readsPrec 0 b in
[(Monom( (read c::Float),((\(Monom(x,y)) -> y) d)), "")]
--Aufruf: (read "2x^2")::Term
main :: IO ()
main = do
putStrLn "Insert a Term:"
inpStr <- getLine
let outStr = (read inpStr)
putStrLn outStr
這將進行編譯/解釋,但是當我調用main並輸入“ 2x ^ 2”時,它僅返回“ 2x ^ 2”而不是Monom(2.0,2)。 如果您致電(閱讀“ 2x ^ 2”):: Term它將正常工作,請問我該怎么做?
通常我會回來
(read inpStr)::Term
而不是putStrLn,因為顯示了讀取工具,但是解釋器讓我發瘋了……任何幫助都將被申請
您可以在let
使用類型限定符。 您的main
看起來像這樣:
main :: IO ()
main = do
putStrLn "Insert a Term:"
inpStr <- getLine
let outStr = read inpStr :: Term
putStrLn $ show outStr
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