extractor-in-crystal.md

Let's consider some value holder. For example, Try monad. For simplicity, Let's assume it only works with Int32.

record Success, value : Int32
record Failure, value : Exception

# some try object
try : Success | Failure = Success(1)

It usually looks like this by using crystal's case statement.

# Extract it.
case try
when Success
  v = try.value
  puts "Got #{v}"
when Failure
  e = try.value
  puts "Oops, #{e}"
end

Generally, we can write intuitively in languages with extractor, such as Scala, Rust, and more recently, Ruby.

Scala

try match {
  case Success(v) => printf("Got  %s\n", v)
  case Failure(e) => ...

Rust

match try {
    Success(v) => println!("Got {:?}", v),
    Failure(e) => ...

Ruby 3

case try
in Success(value: v); puts "Got #{v}"
in Failure(value: e); ...

But we Crystal have powerful macros. Let's define it by myself!

module Try
  macro match(try)
    case {{try}}
    {{yield}}  # To be precise, parse 'when' here and extract the value
    end
  end
end

Try.match(try) {
  when Success(v) then puts "Got #{v}"
}

How elegant! But we will get...

Error: expecting token '}', not 'when'

This style is called Partial Function in Scala. That is first class object and it can be applied to case sentences.

val pf:PartialFunction[Try,Int32] = {
  case Success(v) => printf("Got  %s\n", v)
  case Failure(e) => ...
}

my_try_match(pf) // works

Thus, my wishes are.

1. Crystal will support Extractor. (best)
2. Crystal will support Partial Function for AST. (second best)

Since I had no choice, I now use the local variable TypeDeclaration 😃
https://github.com/maiha/try.cr/blob/master/spec/match_spec.cr#L66-L71