In his book To Mock a Mockingbird, Raymond Smullyan teaches combinatory logic using songbirds in a forest. He derives important results combining various combinators, all using the birds of the enchanted forest. Combinators are an effective tool in designing abstractions with functional programming principles. They are reusable units, make you code very concise, without losing on the expressivity. More than the implementation, the names can go a long way in establishing a common vocabulary of programming idioms and techniques. In an earlier post, I talked about Kestrel, and its implementation in Scala for handling side-effects in an abstraction.
In this post, I look at Thrush, a permuting combinator. A Thrush is defined by the following condition: Txy = yx. Thrush reverses the order of evaluation. Raganwald talks about Thrush and its implementations in Ruby in an excellent post quite some time back.
Why would you want to reverse the order of evaluation in a computation ? Well, if you value readability of your code, and have been cringing at how a mix of expression oriented pipelines and nested function calls can make your code less readable, then a Thrush may be for you.
