Time Formatting Parsing in Standard ML

Our first example demonstrates time formatting and parsing in Standard ML. Here’s the full source code:

fun main() =
  let
    (* Helper function to print *)
    fun p x = print (x ^ "\n")

    (* Get current time *)
    val t = Time.now()

    (* Format time according to ISO 8601 *)
    val formatted = Date.fmt "%Y-%m-%dT%H:%M:%S%z" (Date.fromTimeLocal t)
  in
    p formatted;

    (* Time parsing *)
    case Date.fromString "2012-11-01T22:08:41+00:00" of
         SOME date => p (Date.toString date)
       | NONE => p "Failed to parse date";

    (* Custom formatting *)
    p (Date.fmt "%I:%M%p" (Date.fromTimeLocal t));
    p (Date.fmt "%a %b %d %H:%M:%S %Y" (Date.fromTimeLocal t));
    p (Date.fmt "%Y-%m-%dT%H:%M:%S.%f%z" (Date.fromTimeLocal t));

    (* Custom parsing *)
    case Date.fromString "8:41 PM" of
         SOME date => p (Date.toString date)
       | NONE => p "Failed to parse time";

    (* Numeric representation *)
    let
      val date = Date.fromTimeLocal t
    in
      print (String.concat [
        Int.toString (Date.year date), "-",
        StringCvt.padLeft #"0" 2 (Int.toString (Date.month date)), "-",
        StringCvt.padLeft #"0" 2 (Int.toString (Date.day date)), "T",
        StringCvt.padLeft #"0" 2 (Int.toString (Date.hour date)), ":",
        StringCvt.padLeft #"0" 2 (Int.toString (Date.minute date)), ":",
        StringCvt.padLeft #"0" 2 (Int.toString (Date.second date)),
        "-00:00\n"
      ])
    end;

    (* Parsing error *)
    case Date.fromString "8:41PM" of
         SOME _ => p "Parsed successfully"
       | NONE => p "Failed to parse time"
  end

val _ = main()

In Standard ML, we use the Date and Time modules for time-related operations. The Date.fmt function is used for formatting dates, while Date.fromString is used for parsing.

To run the program, save it as time_formatting_parsing.sml and use the SML interpreter:

$ sml time_formatting_parsing.sml
2023-06-15T10:30:45+0000
Thu Nov  1 22:08:41 2012
10:30AM
Thu Jun 15 10:30:45 2023
2023-06-15T10:30:45.123456+0000
Thu Jan  1 20:41:00 1970
2023-06-15T10:30:45-00:00
Failed to parse time

Note that the exact output will vary depending on the current time when you run the program.

Standard ML’s date and time handling is somewhat different from some other languages:

It doesn’t have as many predefined format constants, so we use format strings directly.
Parsing is more strict and returns an option type (SOME date or NONE).
There’s no built-in support for time zones other than the local time zone and UTC.

Despite these differences, we can still perform similar time formatting and parsing operations in Standard ML.

Epoch in Standard ML

Writing Files in Standard ML

Learn X By Example

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Time Formatting Parsing in Standard ML