Grok Various data structures

From Nemerle Homepage

This page is a part of the Grokking Nemerle tutorial.

Tuples

Tuples are forms of nameless data structures. They are usable when you need to return two or three values from a function.

A tuple is constructed with:

(expr1, expr2, ..., exprN)

and deconstructed with:

def (id1, id2, ..., idN) = expr;

This def thing defines values called id1 through idN and puts respective tuple members into them.

Tuple types are written using the * operator. For example, pair of integers (42, 314) has the type int * int, while ("Zephod", 42.0, 42) has the type string * float * int.

An example follows:

/** Parses time in HH:MM format. */
parse_time (time : string) : int * int
{
  def arr = time.Split (array [':']);
  (System.Int32.Parse (arr[0]), System.Int32.Parse (arr[1]))
}
 
seconds_since_midnight (time : string) : int
{
  def (hours, minutes) = parse_time (time);
  (hours * 60 + minutes) * 60
}
 
foo () : void
{
  def secs = seconds_since_midnight ("17:42");
  ...
}

Another example could be:

// split (3.7) => (3, 0.7)
split (x : double) : int * double
{
  def floor = System.Math.Floor (x);
  (System.Convert.ToInt32 (floor), x - floor)
}

List literals

List literals are special forms of writing lists. Lists are data structures that are used very often in Nemerle - often enough to get their own syntax. Lists in Nemerle are somewhat different than the ArrayList type in .NET.

• the lists in Nemerle are immutable (cannot be changed once created)
• items can be appended at the beginning only (constructing new lists)

Of course you are free to use the .NET ArrayList.

Anyway, to construct a list consisting of a given head (first element) and a tail (rest of elements, also a list), write:

head :: tail

To construct a list with specified elements write:

[ element_1, element_2, ..., element_n ]

This way you can also construct an empty list ([]).