Example: `Indexable`

Indexable is a multi-parameter class in the prelude (so you can use get without defining the class yourself).

Goal

Learn how to pull elements out of common containers with a single API:

lists
arrays

Example uses:

( get 0 [10, 20, 30] , get 2 ["a", "b", "c"] )

How it works

Lists and arrays have Indexable (List a, a) / Indexable (Array a, a) instances.
Tuples use numeric projection like .0 and .1 instead of get.

A more guided example

let
  xs = [10, 20, 30],
  first = get 0 xs,
  third = get 2 xs,
  ys = [1, 2, 3],
  last = get 2 ys
in
  (first, third, last)

get is generally expected to error if the index is out of bounds (depending on the host/runtime implementation). If you need safe indexing (returning Option/Result), write it with match on your container type (e.g. [] vs x::xs for lists).

Worked examples

Example: `head : List a -> Option a`

Problem: return the first list element safely.

let
  head = \xs ->
    match xs
      when [] -> None
      when x::_ -> Some x
in
  (head [] is Option i32, head [10, 20, 30])

Why this works: pattern matching handles empty and non-empty shapes explicitly.

Example: `get` on a larger list

Problem: read specific indices from [100, 200, 300, 400].

let
  xs = [100, 200, 300, 400]
in
  (get 0 xs, get 2 xs, get 3 xs)

Why this works: Indexable (List a, a) provides get for list element access.

Rex