SafeNestedHash

Allows you to populate nested hashes without initializing every level first:

h = SafeNestedHash.new
h[1][2][3] = :foo
h
=> {1 => {2 => {3 => :foo}}}

Don't be tempted by more concise implementations such as can be found at http://www.ruby-forum.com/topic/111524. The fundamental challenge to overcome here is that accessing a non-existent key should not alter the original hash, yet setting a deeply nested hash needs to attach to the original hash (potentially) several levels up. This is accomplished by means of a custom UndefinedHash class which tracks lookup path and reconstructs it if an assignment is found at the end:

h = SafeNestedHash.new
h[1]
=> #<SafeNestedHash::UndefinedHash:0x473d648 @first_key=1, @base={}, @keychain=[]>

h = SafeNestedHash.new
h[1][2][3]
=> #<SafeNestedHash::UndefinedHash:0x473bca8 @first_key=1, @base={}, @keychain=[2, 3]>

h = SafeNestedHash.new
h[1][2][3] = :foo
h[1][2][4][5][6]
=> #<SafeNestedHash::UndefinedHash:0x4737464 @first_key=4, @base={3=>:foo}, @keychain=[5, 6]>
h
=> {1=>{2=>{3=>:foo}}}

To check for existence of a key you must use nil?. Smelly I know, and you can't trivially differentiate from an actual nil stored in the hash, but look Ma, no monkey patching:

h = SafeNestedHash.new
h[1][2][3] = :foo
h[1][2][3].nil?
=> false
h[2].nil?
=> true
h[2][3].nil?
=> true

gtd/safe-nested-hash

SafeNestedHash

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