Making a Read instance in Haskell

Question

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?

Answer 1

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]