Cookbooks
Haskell
Chapter 7. Dates and Times
Converting a date into a string

Haskell:
Converting a date into a string

How to:

In Haskell, you use the formatTime function from the Data.Time.Format module for this job. Let’s dive right into some code:

import Data.Time
import Data.Time.Format (formatTime, defaultTimeLocale)

main :: IO ()
main = do
    -- Grab the current time
    currentTime <- getCurrentTime
    let currentZone = utc
        -- Convert UTC time into a local time object
        localTime = utcToLocalTime currentZone currentTime
        -- Format the date as "YYYY-MM-DD"
        dateString = formatTime defaultTimeLocale "%F" localTime
    putStrLn dateString

And here’s what you might see as output, depending on the current date:

2023-04-01

Deep Dive

Dating back to the early days of programming, converting dates to strings has always been a matter of practical usability. In Haskell, we owe our date and time handling to the Data.Time library, which was inspired by the functionality and improvements over older libraries such as old-time.

There are alternatives to formatTime, like using show to convert a date to a string directly, but this won’t give you custom formatting options. The formatTime function is rich, supporting a variety of formats that align with the C’s strftime function patterns. It’s flexible and locale-aware, using defaultTimeLocale or other locales to format dates according to cultural conventions.

Regarding implementation, the Data.Time.Format functions are pure, meaning they don’t rely on or cause side effects. This aligns with Haskell’s functional programming ethos, which aims for functions to be predictable and their outcomes determined only by their inputs.

See Also

For more extensive work on dates and times in Haskell, peruse the following:

Comparing two datesGetting the current date