RUBY

meaning of {} in ruby

This is probably the most tricky one - {} is also syntax for blocks, but only when passed to a method OUTSIDE the arguments parens.

When you invoke methods without parens, Ruby looks at where you put the commas to figure out where the arguments end (where the parens would have been, had you typed them)

1.upto(2) { puts 'hello' } # it's a block
1.upto 2 { puts 'hello' } # syntax error, ruby can't figure out where the function args end
1.upto 2, { puts 'hello' } # the comma means "argument", so ruby sees it as a hash - this won't work because puts 'hello' isn't a valid hash

meaning of {} in ruby

When on their own, or assigning to a variable, [] creates arrays, and {} creates hashes. e.g.

a = [1,2,3] # an array
b = {1 => 2} # a hash
[] can be overridden as a custom method, and is generally used to fetch things from hashes (the standard library sets up [] as a method on hashes which is the same as fetch)
a = {1 => 2} # create a hash for example
puts a[1] # same as a.fetch(1), will print 2