I have the following type:
data Color = Red
| Yellow
| Green
| ...
I want a function mapping string representation to specific Color.
str2Color :: String -> Color
str2Color "Red": Red
str2Color "Yellow": Yellow
str2Color "Green": Green
I could enumerate all strings, but problem is the list of all colors is very long. Is there any simpler way?
ps: Let's assume all input strings have a corresponding Color, for ease of exposition.
As @WillemVanOnsem noted in a comment, you can have the compiler derive a Read instance for your Color type:
data Color = Red | Yellow | Blue
deriving (Read, Show)
which then makes available the function:
read :: String -> Color
to convert a String to the corresponding Color.
Full example:
data Color = Red | Yellow | Blue
deriving (Read, Show)
main = do
print (read "Red" :: Color)
print (read "Blue" :: Color)
read throws an exception if the String isn’t a valid Color. To handle this case yourself, you can use Text.Read.readMaybe :: (Read a) => String -> Maybe a and pattern-match on the result.
import Text.Read (readMaybe)
main = do
print (readMaybe "Red" :: Maybe Color) -- Just Red
print (readMaybe "Beans" :: Maybe Color) -- Nothing
s <- getLine
case readMaybe s of
Just color -> print (color :: Color)
Nothing -> putStrLn ("Invalid color: '" ++ s ++ "'")
deriving Readfrom theColortype, then you can useread :: String -> Colorto convert a string to the correspondingColor.