3 ms·

The Billy Baldwin of Conditional Assignment in Ruby

54 points by laserlemon 15y ago

pchristensen 15y ago
Stupid name for a blog post, neat Ruby trick. &&= is less well known than ||=. RTFA for examples.
Argorak 15y ago
There is a tiny inaccuracy in this post: @value ||= "foo" is more efficient than @value = @value || "foo" As it actually evaluates as follows: @value || @value = "foo" This is especially good when working with hashes, as the left hand side is only evaluated once, not twice.
- laserlemon 15y ago
  Thank you for pointing this out. Addressed in the comments: http://collectiveidea.com/blog/archives/2011/06/21/the-billy-baldwin-of-conditional-assignment/#comment-form http://collectiveidea.com/blog/archives/2011/06/21/the-billy...
ned 15y ago
Interesting. For the example cited, you could also use ActiveRecord::Dirty def clean_summary self.summary = summary.squish if self.summary_changed? end
- laserlemon 15y ago
  Not quite because the summary could change to nil, but it brings up a good point. There's often a better way in Rails, but &&= is native Ruby so it's especially useful in non-Rails applications.
mofle 15y ago
Does Python have this?
- stdbrouw 15y ago
  Don't think so, but I wouldn't want it anyway, the Ruby syntax being as it is. I think the CoffeeScript equivalents are more intuitive: @current_user ?= User.find() # as opposed to @current_user ||= User.find() and @summary = excerpt?.squish # as opposed to @summary &&= excerpt.squish
- MostAwesomeDude 15y ago
  Python doesn't need it; the savings from not double-evaluating things aren't there. Additionally, Python lore is to expand this idiom to an if statement, for explicitness, especially in polymorphic situations where you actually want to check for "<expr> is None" rather than "bool(<expr>)". Both of these examples are Railsy; equivalent Djangoy statements in form validation would have to deal with None: if data["incoming"] is None: data["incoming"] = "Default value" ... if data["incoming"]: data["incoming"] = data["incoming"].strip() Because of Python's string immutability, there's no point to dodging the assignment; you're still going to incur new string creation anyway.
  - mitchty 15y ago
    I'm sort of torn on using these magic operators in Ruby, and i've used ruby since the 1.4 days. For example this: some_var &&= replace_var() some_var ||= initialize_var() versus a more conventional/idiomatic if: some_var = (some_var) ? replace_var() : initialize_var() ||= turns out to be rather useful as a general assign once operator, but i'm having a hard time justifying use of &&=. That and I find myself after learning Erlang to try to assign only once when possible to make debugging easier. While ||= isn't strictly needed, it makes assigning to class/module variables in a single line easier in general. I'm not sure I agree with the Python "always expand to an if statement" methodology though, it just seems like a bit of unneeded yak shaving. (note: i don't use rails so your post may be specific to web frameworks and I'm off my rocker, if so disregard. >.<)
trustfundbaby 15y ago
I feel like my Ruby brain just grew a little ... very neat Thanks for that.

54 points by laserlemon 15y ago

Stupid name for a blog post, neat Ruby trick. &&= is less well known than ||=. RTFA for examples.

There is a tiny inaccuracy in this post: @value ||= "foo" is more efficient than @value = @value || "foo" As it actually evaluates as follows: @value || @value = "foo" This is especially good when working with hashes, as the left hand side is only evaluated once, not twice.

laserlemon 15y ago
Thank you for pointing this out. Addressed in the comments: http://collectiveidea.com/blog/archives/2011/06/21/the-billy-baldwin-of-conditional-assignment/#comment-form http://collectiveidea.com/blog/archives/2011/06/21/the-billy...

ned 15y ago

Interesting. For the example cited, you could also use ActiveRecord::Dirty def clean_summary self.summary = summary.squish if self.summary_changed? end

laserlemon 15y ago
Not quite because the summary could change to nil, but it brings up a good point. There's often a better way in Rails, but &&= is native Ruby so it's especially useful in non-Rails applications.

mofle 15y ago

Does Python have this?

stdbrouw 15y ago
Don't think so, but I wouldn't want it anyway, the Ruby syntax being as it is. I think the CoffeeScript equivalents are more intuitive: @current_user ?= User.find() # as opposed to @current_user ||= User.find() and @summary = excerpt?.squish # as opposed to @summary &&= excerpt.squish
MostAwesomeDude 15y ago
Python doesn't need it; the savings from not double-evaluating things aren't there. Additionally, Python lore is to expand this idiom to an if statement, for explicitness, especially in polymorphic situations where you actually want to check for "<expr> is None" rather than "bool(<expr>)". Both of these examples are Railsy; equivalent Djangoy statements in form validation would have to deal with None: if data["incoming"] is None: data["incoming"] = "Default value" ... if data["incoming"]: data["incoming"] = data["incoming"].strip() Because of Python's string immutability, there's no point to dodging the assignment; you're still going to incur new string creation anyway.
- mitchty 15y ago
  I'm sort of torn on using these magic operators in Ruby, and i've used ruby since the 1.4 days. For example this: some_var &&= replace_var() some_var ||= initialize_var() versus a more conventional/idiomatic if: some_var = (some_var) ? replace_var() : initialize_var() ||= turns out to be rather useful as a general assign once operator, but i'm having a hard time justifying use of &&=. That and I find myself after learning Erlang to try to assign only once when possible to make debugging easier. While ||= isn't strictly needed, it makes assigning to class/module variables in a single line easier in general. I'm not sure I agree with the Python "always expand to an if statement" methodology though, it just seems like a bit of unneeded yak shaving. (note: i don't use rails so your post may be specific to web frameworks and I'm off my rocker, if so disregard. >.<)

trustfundbaby 15y ago

I feel like my Ruby brain just grew a little ... very neat Thanks for that.