I have a data type
data Time = Time {hour :: Int,
minute :: Int
}
for which i have defined the instance of Show as being
instance Show Time where
show (Time hour minute) = (if hour > 10
then (show hour)
else ("0" ++ show hour))
++ ":" ++
(if minute > 10
then (show minute)
else ("0" ++ show minute))
which prints out times in a format of 07:09.
Now, there should be symmetry between Show and Read, so after reading (but not truly (i think) understanding) this and this, and reading the documentation, i have come up with the following code:
instance Read Time where
readsPrec _ input =
let hourPart = takeWhile (/= ':')
minutePart = tail . dropWhile (/= ':')
in (\str -> [(newTime
(read (hourPart str) :: Int)
(read (minutePart str) :: Int), "")]) input
This works, but the "" part makes it seem wrong. So my question ends up being:
Can anyone explain to me the correct way to implement Read to parse "07:09" into newTime 7 9 and/or show me?
I'll use isDigit and keep your definition of Time.
import Data.Char (isDigit)
data Time = Time {hour :: Int,
minute :: Int
}
You used but didn't define newTime, so I wrote one myself so my code compiles!
newTime :: Int -> Int -> Time
newTime h m | between 0 23 h && between 0 59 m = Time h m
| otherwise = error "newTime: hours must be in range 0-23 and minutes 0-59"
where between low high val = low <= val && val <= high
Firstly, your show instance is a little wrong because show $ Time 10 10 gives "010:010"
instance Show Time where
show (Time hour minute) = (if hour > 9 -- oops
then (show hour)
else ("0" ++ show hour))
++ ":" ++
(if minute > 9 -- oops
then (show minute)
else ("0" ++ show minute))
Let's have a look at readsPrec:
*Main> :i readsPrec
class Read a where
readsPrec :: Int -> ReadS a
...
-- Defined in GHC.Read
*Main> :i ReadS
type ReadS a = String -> [(a, String)]
-- Defined in Text.ParserCombinators.ReadP
That's a parser - it should return the unmatched remaining string instead of just "", so you're right that the "" is wrong:
*Main> read "03:22" :: Time
03:22
*Main> read "[23:34,23:12,03:22]" :: [Time]
*** Exception: Prelude.read: no parse
It can't parse it because you threw away the ,23:12,03:22] in the first read.
Let's refactor that a bit to eat the input as we go along:
instance Read Time where
readsPrec _ input =
let (hours,rest1) = span isDigit input
hour = read hours :: Int
(c:rest2) = rest1
(mins,rest3) = splitAt 2 rest2
minute = read mins :: Int
in
if c==':' && all isDigit mins && length mins == 2 then -- it looks valid
[(newTime hour minute,rest3)]
else [] -- don't give any parse if it was invalid
Gives for example
Main> read "[23:34,23:12,03:22]" :: [Time]
[23:34,23:12,03:22]
*Main> read "34:76" :: Time
*** Exception: Prelude.read: no parse
It does, however, allow "3:45" and interprets it as "03:45". I'm not sure that's a good idea, so perhaps we could add another test length hours == 2.
I'm going off all this split and span stuff if we're doing it this way, so maybe I'd prefer:
instance Read Time where
readsPrec _ (h1:h2:':':m1:m2:therest) =
let hour = read [h1,h2] :: Int -- lazily doesn't get evaluated unless valid
minute = read [m1,m2] :: Int
in
if all isDigit [h1,h2,m1,m2] then -- it looks valid
[(newTime hour minute,therest)]
else [] -- don't give any parse if it was invalid
readsPrec _ _ = [] -- don't give any parse if it was invalid
Which actually seems cleaner and simpler to me.
This time it doesn't allow "3:45":
*Main> read "3:40" :: Time
*** Exception: Prelude.read: no parse
*Main> read "03:40" :: Time
03:40
*Main> read "[03:40,02:10]" :: [Time]
[03:40,02:10]