haskell - Haskell 在带有类型的 main 中使用实例化读取

Question

如何在我的 main 中使用实例化读取？

现在我有以下内容：

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    

这将编译/解释，但是当我调用我的 main 并输入“2x^2”时，它只返回“2x^2”而不是 Monom(2.0,2)。如果您调用 (read "2x^2")::Term 它会正常工作，我该怎么做呢？

通常我会回来

(read inpStr)::Term  

而不是 putStrLn 因为读取实现显示但解释器让我发疯......任何帮助都会被应用

Answer 1

您可以在let. 你main看起来像这样：

main :: IO ()
main = do
   putStrLn "Insert a Term:"
   inpStr <- getLine
   let outStr = read inpStr :: Term
   putStrLn $ show outStr