Syntax: special handling of tuple condition in case expression

[asterite]

This is a follow up of #2239

Thanks to suggestions from @kostya and @ysbaddaden, I changed the proposed syntax from this:

case exp1, exp2, ..., expN
when cond1, cond2, ..., condN
  body
end

To this:

case {exp1, exp2, ..., expN}
when {cond1, cond2, ..., condN}
  body
end

As you can see, that syntax is already supported by the current compiler, so the compiler is just updated to rewrite the code in a few special cases. Specifically, if the case condition is a tuple literal, then:

• If a when condition is a tuple literal (starts with {), then the "implicit object" notation is allowed in its members (for example {.even?, .odd?})
• If a when condition is a tuple literal, a compile error is issued if its size is not that of the condition (otherwise you are checking a case that will always be false)
• If a when condition is a tuple literal, the comparison made against the case tuple expression is done in a smarter way, similar to how a regular case is handled. For example, case {x, y}; when {Int32, Float64} is rewritten to if x.is_a?(Int32) && y.is_a?(Float64), allowing type filters to work, something that doesn't work right now (this is the main motivation for adding this funcionality).
• If a when condition is not a tuple literal, the regular === is used. For this to work, I added Tuple#===, which was missing.

With this, we can finally solve real world problems like fizzbuzz in an elegant way:

100.times do |i|
  case {i % 3, i % 5}
  when {0, 0}
    puts "FizzBuzz"
  when {0, _}
    puts "Fizz"
  when {_, 0}
    puts "Buzz"
  else
    puts i
  end
end

Of, if you want {0, 0} as a constant, you can do it too:

ZEROS = {0, 0}

100.times do |i|
  case {i % 3, i % 5}
  when ZEROS
    puts "FizzBuzz"
  when {0, _}
    puts "Fizz"
  when {_, 0}
    puts "Buzz"
  else
    puts i
  end
end

Another advantage of this syntax is that it allows multiple when conditions, for example:

case {x, y}
when {0, 0}, {1, 1}
  # ...
end

[sdogruyol]

Wow, this is awesome. Feels like pattern matching here 👍

[ysbaddaden]

Strong 👍

[waterlink]

Definite 👍 from my side. I feel that is very empowering feature.

Best Regards,
Oleksii Fedorov,
Competent Programmer,
TDD, Microservices, Ruby, Clojure, Go and more,
+49 15757 486 476

On Wed, Mar 2, 2016 at 9:37 PM, Julien Portalier [email protected]
wrote:

Strong [image: 👍]

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#2258 (comment)
.

[asterite]

One thing that @bcardiff told me to remark is that this won't work:

foo = {1, 2}
case foo
when {.even?, .odd?} # Syntax error: expected token: .
end 

The special rules only apply if the case expression is a tuple literal.

If it's a tuple literal, the syntax desugaring can be done before semantic analysis. Otherwise, in the case above, the compiler would first need to check if foo is a Tuple of that size and then do the corresponding checks. This is something that could be (more or less easily) done in the future, though. Basically rewrite it to:

if foo.is_a?(Tuple(_, _)) && foo[0].even? && foo[1].odd?
  # ...
end

Problem is, .is_a?(Tuple(_, _)) doesn't compile right now, but should be easily fixable.

Note however, that this does work:

case foo
when {1, 2}
  # ...
end

because nothing special (neither .even? nor _) was used in the tuple.