additionally you want to separate the bmi-calculator from the main function as it is a pure function without any side-effects
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)
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solved parse error in haskell <- [closed]