Feature #20769: Add `Hash#transform_value`

• Status: Open
• Assignee: -
• Target version: -

[seanpdoyle (Sean Doyle)]

Add Hash#transform_value as a specialized, key-specific version of Hash#transform_values.

hash = { image: "https://example.com/image.jpg" }
mutated_hash = hash.transform_value(:image) { |url| download(url) }

hash # => { image: "https://example.com/image.jpg" }
mutated_hash # => { image: File<...> }

hash.transform_value!(:image) { |url| download(url) }
hash # => { image: File<...> }

Similar value transformation can be achieved through variable assignment and direct mutation:

hash = { image: "https://example.com/image.jpg" }

hash.merge(image: download(hash[:image]))

hash[:image] = download(hash[:image])

While simple and currently supported, it requires a local variable (and therefore poses some challenges when chaining other methods) and repeats the Hash key in both the reading and writing portions of the code.

[nobu (Nobuyoshi Nakada)]

hash.merge(image: nil) {|_, url| download(url)}

hash.merge!(image: nil) {|_, url| download(url)}

[seanpdoyle (Sean Doyle)]

Thank you for sharing that code sample. It demonstrates an interesting way to use Hash#merge with a block argument.

When the key argument is absent from the Hash, a call to Hash#transform_value would be a no-operation. I apologize for not including this in the original example:

hash  = { foo: "value" }
transformed_hash = hash.transform_key(:bar) { |value| fail "This block is never yielded" }
transformed_hash # => { foo: "value" }

nobu (Nobuyoshi Nakada) wrote in #note-1:

hash.merge(image: nil) {|_, url| download(url)}

hash.merge!(image: nil) {|_, url| download(url)}

Using Hash#merge in the way you have shared does reduce the number times the key name is repeated.

However, without a guarding conditional call to hash.key?(:image), the returned Hash would include image: nil when an :image key was not initially present:

hash = {}
transformed_hash = hash.merge(image: nil) { |_, url| download(url) }
transformed_hash # => { image: nil }

hash = {}
transformed_hash = hash.merge(image: nil) { |_, url| download(url) } if hash.key?(:image)
transformed_hash # => {}