Like formal theories of language, functional theories of language come from the field of linguistics, although this type of linguistics is less commonly found in the U.S.. The type of functional linguistics I am most familiar with is Systemic Functional Linguistics, based on the work of Michael Halliday, and practiced predominantly in Australia. While I am also far from an expert in functional linguistics, a key tenet of functional linguistics is that language cannot be separated from social function, or what humans do with language. This means that in contrast to formal linguistics, there is not a linguistic system that can be analyzed independently of the way it is deployed. In terms of language learning and teaching, this means that:
1) Language learning occurs through repeated performance of language functions. In contrast to formal linguistics, there is no internal language learning mechanism, but simply repeated exposure to language in a social function. This appears to me to be in many ways similar to usage-based approaches of language acquisition, although I’m sure there are nuances I am unfamiliar with here. The key to learning is contextualized use of the language in social functions, rather than learning individual vocabulary items or grammatical concepts.
2) Linguistic elements cannot be taught separately from social functions. This principle places pragmatic, sociolinguistic, and grammatical features, as well as vocabulary, all at the same level, rather than viewing pragmatic and sociolinguistic features as extra or advanced, or calling for a buildup from vocabulary, to grammar, to pragmatics and sociolinguistics. The implication of this is that it is essential to address all linguistic features from the earliest stages of language learning, rather than focusing on vocabulary and grammar in beginning classes, and pragmatic and sociolinguistic features in advanced ones.
3) The social function, rather than specific linguistic features, are the primary goal of language learning. Similar to the point above, the goal of a lesson, curriculum, or unit is the ability to perform certain functions in the language, something that fits in well with the “Can-Do Statements” developed by organizations such as the American Council on the Teaching of Foreign Languages, and the Common European Framework of Reference. It is worth noting that depending on the context, certain language functions do not require complete grammatical accuracy. If I want to rent an apartment, and ask for a شقة كبير instead of a شقة كبيرة ,missing the gender agreement, I will probably still get the apartment. On the other hand, if I’m presenting at an academic conference, failing to perform gender agreement could make the audience take my message less seriously. In both situations, the emphasis is on what is necessary to do the social function, rather than the linguistic elements themselves.
In my experience, despite the introduction of Can-Do Statements by ACTFL in 2013, functional theories of language are rarely the basis of language pedagogy in the United States, it is much more common to see formal or folk linguistic theories of language informing pedagogy. One exception is work on genre-based approaches to pedagogy, which I personally find quite appealing, and this is why I am working to use them our curriculum. Yet again, I think the most important thing is to be aware of what our theories of language are, and if these are the theories and ideologies we want to perpetuate. Once we decide this, we can examine the extent to which our classroom practice reflects and perpetuates various ideologies, and decide what changes we need to make.