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Parser: allow anonymous block args #8117

merged 1 commit into from Sep 5, 2019


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commented Aug 25, 2019

Related to #7157

In addition to being able to write this:

def foo(x, y, &block)

Now you can write it like this:

def foo(x, y, &)

That is, you can omit the block name.

Of course it also works with a type signature:

def foo(x, y, & : Int32 -> Int32)

It might look more obscure but there's a reason we might go this way.


Right now the compiler infers a few things based on the signature and method definition:

  • If a method has yield inside it the compiler knows it receives a block
  • If a method mentions the block argument then the compiler considers the block is captured and it will form a closure over it when invoked

This is nice but it has a few disadvantages:

  • because you can just use yield and omit the block argument it's not immediately clear from the method signature whether a method accepts a block or not
  • you must use &block if you want to say you receive a block, even if you don't plan to yield (Nil#try does this) so it's a bit asymmetric that sometimes you need to write it and sometimes not
  • if yield is generated through a macro then you are again forced to specify the block argument
  • if the block argument is mentioned inside a macro then the compiler doesn't know it actually forms a closure (#2992) and one has to mention the block argument somewhere outside the macro code as an ugly workaround

So the idea would be:

  • Just & means a non-captured block argument: because it doesn't have a name there's no way to capture it
  • &block means a captured block argument, and the compiler will verify that it's used in the semantic phase (because it might be mentioned inside a macro)
  • &block without a mention will give a compile error suggesting to either change it to & to avoid a closure or to mention it somehow
  • if you yield and the method signature doesn't have & the compiler will tell you

Now, this PR is just the beginning direction: there's no breaking change here, just the possibility to omit the block argument name.

@asterite asterite merged commit b9c989e into crystal-lang:master Sep 5, 2019
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@asterite asterite deleted the asterite:anonoymous-block-arg branch Sep 5, 2019
@asterite asterite added this to the 0.31.0 milestone Sep 5, 2019
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