Hamstar.update_having is a module_function that works just like Hamster update_in() but with three additional ways to select container elements:

1. the Kleene star operator denoted by '*'
2. the associative selector denoted by an array containing a key and a value e.g. [:name,'Chris'] your values will be compared to this one using the case comparison operator === so you can use Strings or regexps or other things that define === (such as classes and Ranges)
3. generalized Proc-based matching e.g. you can supply a lambda directly in the path specification

Quick-start instructions follow. For more background, see: Hamstar Transforms Immutable Ruby Collections Better.

Add this line to your application's Gemfile:

gem 'hamstar'

And then execute:

$ bundle

Or install it yourself as:

$ gem install hamstar

With plain old update_in() you can:

require 'hamster'
x = Hamster.from([{name:'Chris', hobbies:['clarinet']},{name:'Pat',hobbies:['bird watching','rugby']}])
x.update_in(1,:name){|name| 'Patsy'}
=> Hamster::Vector[Hamster::Hash[:hobbies => Hamster::Vector["clarinet"], :name => "Chris"], Hamster::Hash[:hobbies => Hamster::Vector["bird watching", "rugby"], :name => "Patsy"]]

You can do the exact same thing with Hamstar's update_having():

require 'hamstar'
Hamstar.update_having( x, 1,:name){|name| 'Patsy'}
=> (same result as before)

And you can go further, and replace the vector offset 1 with the Kleene star '*' to operate on all elements of the vector:

Hamstar.update_having( x, '*',:name){|name| name + 'sy'}
=> Hamster::Vector[Hamster::Hash[:name => "Chrissy", :hobbies => Hamster::Vector["clarinet"]], Hamster::Hash[:name => "Patsy", :hobbies => Hamster::Vector["bird watching", "rugby"]]]

And what if you wanted to efficiently replace every 'Pat' with 'Patsy', without having to add conditional code to your block? Hamstar let's you use a key/value pair as part of your path specification:

Hamstar.update_having( x, [:name,'Pat'],:name){|name| 'Patsy'}
=> Hamster::Vector[Hamster::Hash[:name => "Chris", :hobbies => Hamster::Vector["clarinet"]], Hamster::Hash[:name => "Patsy", :hobbies => Hamster::Vector["bird watching", "rugby"]]]

Note that your values will be compared to the second element of the pair using the case comparison operator ===. That means you can use a Regexp there (or any other object that defines ===) e.g.:

Hamstar.update_having( x, [:name,/P/],:name){|name| name+'sy'}
=> (same result as before)

Finally, if none of the options given above work for you, you can use an arbitrary Proc as a matcher. Here's an example that supplies a lambda inline:

Hamstar.update_having( x, ->(k,v){k.odd?},:name){|name| 'Patsy'}
 => Hamster::Vector[Hamster::Hash[:name => "Chris", :hobbies => Hamster::Vector["clarinet"]], Hamster::Hash[:name => "Patsy", :hobbies => Hamster::Vector["bird watching", "rugby"]]]

See hamstar_spec.rb for more examples.

1. Fork it ( http://github.com/Bill/hamstar/fork )
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request