Higher order message

A higher order message (HOM) in a computer programming language is a form of higher-order programming that allows messages that have other messages as arguments. The concept was introduced at MacHack 2003 by Marcel Weiher and presented in a more complete form in 2005 by Marcel Weiher and Stéphane Ducasse. Loops can be written without naming the collections looped over, higher order messages can be viewed as a form of point-free or tacit programming.

Examples
In ordinary Smalltalk code, without using HOM, obtaining a collection of the employees that have a salary of 1000 would be achieved with the following code: However, using HOM, it can be expressed as follows: Here,  is a higher order message, and   is understood to be called on the select message itself, rather than on its result. The Smalltalk language was not modified to implement this feature. Instead,  returns a message that reifies the   send, which then interprets the   message.

Another example is the use of future message sends in the Croquet Project: In this example, the  message causes the   message to be sent to the cube object after 1 second.

Adoption
The reference implementation in Objective-C leverages the trait that in Objective-C, objects that don't understand a message sent to them, still get it delivered in a special hook method, called. Higher order messaging was implemented in a number of languages that share this feature including Ruby and Smalltalk.

ECMAScript Harmony's Proxies documentation specifically mentions higher order messages as an application for their Catchall Proxies.

Similar concepts in other languages
The programming language J distinguishes between verbs and adverbs. Adverbs modify the functioning of verbs. This is similar to higher order messages (the adverbs) modifying the messages that follow (the verbs). In the Croquet example above, the. message is still sent and has its normal meaning, but its delivery is modified by the   message, it will be sent sometime in the future.