A higher order message (HOM) in a computer
programming language
A programming language is a system of notation for writing computer programs.
Programming languages are described in terms of their Syntax (programming languages), syntax (form) and semantics (computer science), semantics (meaning), usually def ...
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
Smalltalk is a purely object oriented programming language (OOP) that was originally created in the 1970s for educational use, specifically for constructionist learning, but later found use in business. It was created at Xerox PARC by Learni ...
code, without using HOM, obtaining a collection of the employees that have a salary of 1000 would be achieved with the following code:
salaried := employees select: each hasSalary: 1000
However, using HOM, it can be expressed as follows:
salaried := employees select hasSalary: 1000.
Here,
select is a higher order message, and
hasSalary: 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,
select returns a message that
reifies the
select send, which then interprets the
hasSalary: message.
Another example is the use of future message sends in the
Croquet Project
The Croquet Project is a software project that was intended to promote the continued development of the Croquet open-source software, open-source software development kit to create and deliver collaborative multi-user online application software ...
:
(cube future:1000) addRotationAroundY:10.
In this example, the
future: message causes the
addRotationARoundY: message to be sent to the cube object after 1 second.
Adoption
The reference implementation in
Objective-C
Objective-C is a high-level general-purpose, object-oriented programming language that adds Smalltalk-style message passing (messaging) to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was ...
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
forward:. Higher order messaging was implemented in a number of languages that share this feature including
Ruby
Ruby is a pinkish-red-to-blood-red-colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapph ...
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
verb
A verb is a word that generally conveys an action (''bring'', ''read'', ''walk'', ''run'', ''learn''), an occurrence (''happen'', ''become''), or a state of being (''be'', ''exist'', ''stand''). In the usual description of English, the basic f ...
s and
adverb An adverb is a word or an expression that generally modifies a verb, an adjective, another adverb, a determiner, a clause, a preposition, or a sentence. Adverbs typically express manner, place, time, frequency, degree, or level of certainty by ...
s. 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
addRotationAroundY:. message is still sent and has its normal meaning, but its delivery is modified by the
future:1000 message, it will be sent sometime in the future.
References
{{reflist, 2
Programming paradigms