pattern.cr

Structural pattern matching proof-of-concept library

Usage

Due to syntax restrictions, macro arguments must be valid Crystal expressions and cannot declare new variables, and features like splats in container literals are not supported (but see crystal-lang/crystal#3718). The following conventions are applied in this library:

• exp ~>? pat becomes Pattern.matches?(exp, pat). Flow typing will not work properly because the bound variables in pat must be declared prior to this point; use Pattern.try_match(exp, pat) in those cases instead.
• exp ~> pat becomes Pattern.match!(exp, pat). Flow typing works here due to the exception thrown in the failing branch.
• *x in array patterns are replaced with __splat(x). The __splat doesn't need to be an existing method.
• **x in hash patterns are replaced with __double_splat => x. Note that NamedTuple patterns cannot have a double splat in this way.

Refer to the same examples in SPEC.md to see how compiler support for structural pattern matching would look like. See also the known limitations.

# Before
ary = [1, 2, 3, 4, 5, 6, 7]
if ary.size >= 2
  a, b = ary
  rest = ary[2..]
else
  raise "not enough elements"
end

# After
require "pattern"
a, b, rest = nil, nil, nil
Pattern.match! [1, 2, 3, 4, 5, 6, 7], [a, b, __splat(rest)]

# Before
if ary.size == 3
  first, x, y = ary
end
if first.is_a?(Circle) && x.is_a?(Int32) && y.is_a?(Int32)
  first.move_to(x, y)
end

# After
require "pattern"
first = nil
x = nil
y = nil
Pattern.try_match(ary, {first = Circle, x = Int32, y = Int32}) do
  first.move_to(x, y)
end

# Before
anniversary = events.find do |ev|
  ev.has_key?("start") && ev["start"].in?("2020-01-01".."2020-12-31") &&
    ev.has_key?("id") && ev["id"] =~ /^ANNIV.*/
end["start"]
events.select! &.[]?("start").== anniversary

# After
require "pattern"
anniversary = nil
Pattern.match!(events, {__splat(_), {
  "start" => anniversary = "2020-01-01".."2020-12-31",
  "id" => /^ANNIV.*/,
}, __splat(_)})
events.select! do |ev|
  Pattern.matches?(ev, {"start" => __pin(anniversary)})
end

Installation

1. Add the dependency to your shard.yml:

   dependencies:
     pattern:
       github: HertzDevil/pattern.cr

2. Run shards install

Contributing

1. Fork it (https://github.com/HertzDevil/pattern.cr/fork)
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

Contributors

• Quinton Miller - creator and maintainer