Feature #15814
closed
Capturing variable in case-when branches
Description
In ruby, when a case-when statement is written, the when branches accepts expressions which will be evaluated to objects, then === is called to check if any of them returns true:
case 'a'
when 'abc'
# not matched
when Regexp.new('[abc]')
puts :matched # => matched
end
To demonstrate what is being done here, this is a mock:
equal_all_mock = Object.new
class << equal_all_mock
def ===(anything) true end
end
# 1
case 'a'
when equal_all_mock
puts :matched # => matched
end
# 2
if equal_all_mock === 'a'
puts :matched # => matched
end
Often times when matching for conditional statements, they have values in addition to being truthy or falsey; for example, it is very tempting to write (bugged) code like this (context: parsing 2D robot path instructions):
case
when i = '^v<>'.index(code)
x += [0, 0, -1, 1][i]
y += [1, -1, 0, 0][i]
when code = '/\\'[code]
if code == '/'
dx, dy = dy, dx
else
dx, dy = -dy, -dx
end
when code == '#'
dx = -dx
dy = -dy
end
This pattern has problems:
-
Using assignment to capture expressions "leaks" the local variable into the current scope, which the case block doesn't lock into a block scope, as it's not a proc
-
Even if the match fails, the expression is still written;
code = '/\\'[code]in this case may assign nil, of which thencode == '#'will fail -
The alternative would be using regex, such as
/\^v<>/and then using$&to fetch match data... but the global variable pattern is said to be discouraged, and while it works in this specific case it doesn't work in others, like if I want to act upon the index of an array search (but not when the search result is nil)
Thus my proposal:
case
when '^v<>'.index(code) => i
x += [0, 0, -1, 1][i]
y += [1, -1, 0, 0][i]
when '/\\'[code] => code
if code == '/'
dx, dy = dy, dx
else
dx, dy = -dy, -dx
end
when code == '#'
dx = -dx
dy = -dy
end
This is based on the rescue Exception => e syntax. The when expression => i format could potentially even be extended to:
case 'foobar'
when /fo./ => match
p match # => foo
end
or with a proc that accepts 0~1 parameters (if it expects one, ruby could feed in the truthy value):
case
when '^v<>'.index[code] do |i|
x += [0, 0, -1, 1][i]
y += [1, -1, 0, 0][i]
end
when '/\\'[code] do |code|
if code == '/'
dx, dy = dy, dx
else
dx, dy = -dy, -dx
end
end
when code == '#'
dx = -dx
dy = -dy
end
While some cases like these could be replaced by if-else statements I feel like this would be much better as an enhancement on the pattern-matching side. Scala, for example, does have case x if x % 15 == 0 => { statements } in its pattern-matching; handy when writing fizzbuzz.
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