Add a toFormat function

[lohmander]

I'd like to have the option to specify the output format of a date string. However I am not sure if it is appropriate for a library specifically targeted towards ISO8601.

What I had in mind was using some format similar to http://strftime.org/

ISO8601.toFormat "%H:%M" time       -- => "12:05"
ISO8601.toFormat "%-d/%-m %I:%M%p"  -- => "16/4 03:45PM"

I'd be willing to take a look at a basic implementation if you/we think this is worth doing. I think it'd be enough to just implement a subset of the different formatting variables. I think all of the named days and months could the left out (since these would preferably be available in multiple languages if added).

[jweir]

At first I considered adding a toFormat method. But then what format: C strftime, Golang, YYYY-MM-DD, etc?

More than once I have made a mistake with strftime using an M when I wanted an m or vice-versa.

Using a function with the Time record it would be clear what the expected format is.

So instead of ISO8601.toFormat "%H:%M" time your would formatHourMinute time where

formatHourMinute : Time -> String
formatHourMinute time =
    String.join ""
        [ fmt time.hour
        , ":"
        , fmt time.minute
        ]

What about an Extras package that includes some helper functions to aid in formatting? Then authors can quickly make their own toString, toFormat, etc functions.

WDYT?

[lohmander]

Hm, yeah that's a valid point for sure.

I think an Extras package sounds like a good idea. I'm not sure if this is just me being overly enthusiastic about function composition, but how about making a bunch of helpers to compose your own format like you said, but with an API looking something like this

formatHourMinute : Time -> String
formatHourMinute =
    formatter (hour >> str ":" >> minute)

formatYMD : Time -> String
formatYMD =
    formatter (year >> str "-" >> month >> str "-" >> day)

What do you think? I think it looks quite nice and clear. Or would you prefer a more "manual" approach?

[jweir]

I am not going to have time to implement anything in the near term. Do you want to take a stab at a simple formatting package that solves your problem(s)? At the very least you should have something useful for yourself.

I created an experimental branch with the formatting code in ISO8601.Formatter (it is also simpler) https://github.com/jweir/elm-iso8601/tree/exp/formatter

[lohmander]

@jweir I replied to your comments on the PR 🍻

[jweir]

@lohmander at this point I am not going to add the formatting. I am starting to finally use this package for a real app. As I do, and have this package interact with other packages, I hope to learn more about its flaws and what it needs.

Also, my inclination is that Elm core should provide a Date (or DateTime) record like the one iso-8601 produces. Then package authors can have a standard model to work with for formatters, parsers, localization, calendars, datepickers, etc.

[jweir]

Ok, merged your formatter into a new branch track/formatter.

I made some changes and hid the underlying Time record. So its fields are no longer publicly accessible. This means having a formatter is much more important (my prior method of formatting strings don't work anymore).

[lohmander]

@jweir Nice! How do you wanna proceed with this feature now?