Example: Custom Show type class

This mirrors rex/examples/typeclasses_custom_show.rex.

Goal

Define your own show-printing API that turns values into string without baking formatting into every call site.

We’ll build it up in layers:

1. define the class
2. add a base instance (i32)
3. add a structured type (Point)
4. (optional) add a container instance (List a)

class DemoShow a
  demo_show : a -> string

type Point = Point { x: i32, y: i32 }

instance DemoShow i32
  demo_show = \_ -> "<i32>"

instance DemoShow Point
  demo_show = \p -> "Point(" + demo_show p.x + ", " + demo_show p.y + ")"

demo_show (Point { x = 1, y = 2 })

Extending it

Add an instance DemoShow (List a) <= DemoShow a (see rex/examples/typeclasses_custom_show.rex) and call demo_show [Point { x = 1, y = 2 }].

A worked DemoShow (List a) instance

Here is the list instance from the repo example, with commentary:

class DemoShow a
  demo_show : a -> string

instance DemoShow i32
  demo_show = \_ -> "<i32>"

instance DemoShow (List a) <= DemoShow a
  demo_show = \xs ->
    let
      step = \out x ->
        if out == "["
          then out + demo_show x
          else out + ", " + demo_show x,
      out = foldl step "[" xs
    in
      out + "]"

Why the <= DemoShow a constraint?

Because the implementation calls demo_show x for list elements, so it requires DemoShow a.

Worked examples

Example: use "; " as the list separator

Problem: format list output with semicolons.

class DemoShow a
  demo_show : a -> string

instance DemoShow i32
  demo_show = \_ -> "<i32>"

instance DemoShow (List a) <= DemoShow a
  demo_show = \xs ->
    let
      step = \out x ->
        if out == "["
          then out + demo_show x
          else out + "; " + demo_show x,
      out = foldl step "[" xs
    in
      out + "]"

demo_show [1, 2, 3]

Why this works: only the separator string changed; the fold structure stays the same.

Example: DemoShow bool

Problem: add show-print support for booleans.

class DemoShow a
  demo_show : a -> string

instance DemoShow bool
  demo_show = \b -> if b then "true!" else "false!"

(demo_show true, demo_show false)

Why this works: the instance defines one method body specialized to bool.

Example: DemoShow (Option a)

Problem: print Some(...) and None for options.

class DemoShow a
  demo_show : a -> string

instance DemoShow i32
  demo_show = \_ -> "<i32>"

instance DemoShow (Option a) <= DemoShow a
  demo_show = \ox ->
    match ox
      when Some x -> "Some(" + demo_show x + ")"
      when None -> "None"

(demo_show (Some 1), demo_show None)

Why this works: pattern matching distinguishes constructors and delegates formatting of payloads.