解析错误< - 在 weight<-getLine 发生

Question

--bmicalculator 
bmicalculator::IO() 
bmicalculator=do bmicalculator::(RealFloat a)=>a->a->String 
putStrLn "Please Input your weight" 
weight<-getLine 
putStrLn "Please Input your height" 
height<-getLine 
|bmi<=17.5="You are anorexia!" 
|bmi<=20.7="You are Under weight" 
|bmi<=26.4="You are in normal range" 
|bmi<=27.8="You are marginally overweight" 
|bmi<=31.1="You are overweight" 
|bmi>31.1="You are super OBESE!!" 
where bmi=weight/height^2 

错误我怎么能提示用户输入“重量”和“高度”，然后计算和像我创建的数据类型返回字符串bmicalculator::(RealFloat a)=>a->a->String解析错误< - 在</p> <pre><code>weight<-getLine </code></pre> <p>发生

Answer 1

另外要的BMI-计算器从主功能分开，因为它是没有任何副作用的纯函数

main :: IO() 
main = do 
    putStrLn "Please Input your weight" 
    w <- getLine 
    let weight = read w :: Float 
    putStrLn "Please Input your height" 
    h <- getLine 
    let height = read h :: Float 
    putStrLn $ bmicalc weight height 

bmicalc :: Float -> Float -> String 
bmicalc weight height | bmi<=17.5 = "You are anorexic!" 
         | bmi<=20.7 = "You are underweight" 
         | bmi<=26.4 = "You are in normal range" 
         | bmi<=27.8 = "You are marginally overweight" 
         | bmi<=31.1 = "You are overweight" 
         | otherwise = "You are super OBESE!!" 
         where bmi=weight/(height*height) 

Answer 2

首先你的缩进是错误的（也就是说：不存在）。 do-block的内容应该缩进。

然后你给出了两种不同于bmicalculator的类型签名。第二个是在它明显不属于的do-block内部。

然后，你似乎在do-block内部使用模式防护，在任何模式匹配构造之外。这在语法上无效。那你可能错过了case bmi of。您还需要将=替换为->。

最后，您不能使用位于do - 块之后的where-block内的do-block的本地变量。您应该使用do - 块内的let。此外weight和height是字符串，所以你不能对它们进行算术而不先把它们转换成数字。