Difference between {} and do/end

I'm coming back to Ruby after a couple years away, and finding I've forgotten some of the basics. What's the difference between { ... } blocks and do ... end blocks? In particular, why does one of the following work, while the other fails?

'%61lice'.gsub! /%(\d+)/ do $1.to_i(16).chr end # => "alice"
'%61lice'.gsub! /%(\d+)/ { $1.to_i(16).chr } # => SyntaxError

It has to do with operator precedence. {...} has high precedence,
while do ... end has low precedence. So in your examples above, the
do...end block is pased to the gsub! method, while the {...} block is
passed to the regex. You can fix the second example using parens:

'%61lice'.gsub!(/%(\d+)/) { $1.to_i(16).chr } => "alice"

···

On Feb 20, 11:25 am, Jeff Schwab <j...@schwabcenter.com> wrote:

I'm coming back to Ruby after a couple years away, and finding I've
forgotten some of the basics. What's the difference between { ... }
blocks and do ... end blocks? In particular, why does one of the
following work, while the other fails?

'%61lice'.gsub! /%(\d+)/ do $1.to_i(16).chr end # => "alice"
'%61lice'.gsub! /%(\d+)/ { $1.to_i(16).chr } # => SyntaxError

Jeff Schwab wrote:

I'm coming back to Ruby after a couple years away, and finding I've forgotten some of the basics. What's the difference between { ... } blocks and do ... end blocks?

Another difference is that {} can be confused with a hash literal. I can't think of a good example where it's actually ambiguous, though.

karlvonl wrote:

···

On Feb 20, 11:25 am, Jeff Schwab <j...@schwabcenter.com> wrote:

I'm coming back to Ruby after a couple years away, and finding I've
forgotten some of the basics. What's the difference between { ... }
blocks and do ... end blocks? In particular, why does one of the
following work, while the other fails?

'%61lice'.gsub! /%(\d+)/ do $1.to_i(16).chr end # => "alice"
'%61lice'.gsub! /%(\d+)/ { $1.to_i(16).chr } # => SyntaxError

It has to do with operator precedence. {...} has high precedence,
while do ... end has low precedence. So in your examples above, the
do...end block is pased to the gsub! method, while the {...} block is
passed to the regex. You can fix the second example using parens:

'%61lice'.gsub!(/%(\d+)/) { $1.to_i(16).chr } => "alice"

Thank you!

David Masover wrote:

Jeff Schwab wrote:

I'm coming back to Ruby after a couple years away, and finding I've forgotten some of the basics. What's the difference between { ... } blocks and do ... end blocks?

Another difference is that {} can be confused with a hash literal. I can't think of a good example where it's actually ambiguous, though.

Isn't

p {1=>2}

such an example? The parser decides that it's a block and barfs on the =>.

···

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