Historically, what kind of thing did a form language do?

In earlier chapters, I have presented a conception of living process, in which sequences of steps — each step creating centers — allow a person or a group of people to create a building, or a part of a building, with reasonable expectation that it will be well adapted to its context, and will have living structure.
The patterns in the sequence provide the geometric content which is specific to purpose, culture, climate, and so on. The sequence is organized so that the form unfolds coherently, and allows all the necessary patterns to appear in the finished structure.
Traditional sequences, however, went further. In traditional society, the sequences which people used to make their buildings also created the form, the geometry, the style, so that the entire building production of a society had a deep coherence of style and substance, with the result that buildings and outdoor spaces and their details, cohered together to form a living, geometric unity.
[…] The pattern languages and sequences in use in traditional societies, nearly always specified form, shape, volume, material, style, ornament. They embodied a coherent geometric, visual, physical style, so that now, long afterwards, the architecture of each given place and time, is still recognizable as part of the geometric living unity which was created by that culture.

In addition to a hierarchy of scales, there is another hierarchy in play here: patterns are more abstract and can be implemented with different shapes and materials. Below that, the form needs to make decisions about which material is going to be used and what size it will ultimately be so that it has a certain style.

I shall keep on calling such geometric languages, which create building form and style as well, not merely pattern languages, but form languages.

The idea of pattern languages was inspired by traditional languages, and drew much of their structure and content from traditional languages. However, the emphasis I gave, in the published pattern languages, was to the functional content of the patterns which the language dealt with, and created.
In the late 1960s and early 1970s, the content of architecture was so seriously damaged, that it was not only marked by a breakdown of form, but more horribly, by an almost total breakdown of sensible human content. It was vital, at that time, to focus on the objective nature of the patterns required for comfort (and life) in human surroundings: and to find ways of making this content visible and usable.
The task of doing this was so urgent, and so massive, that my collaborators and I spent six or seven years, merely accumulating material to undertake this task — and in so doing we neglected the equally important task of finding publicly accessible ways in which the actual geometric form of buildings could be unfolded, successfully, from the patterns.

It seems the goal of pattern languages is equivalent to the goal of providing a great user experience in software. Form languages do provide more than that and also describe how to achieve that experience by providing common building blocks to combine.

What in Alexander’s theory is equivalent to a great developer experience then?

The creation of living structure in a society, cannot rest on the living process alone, as I have described it thus far. The emergence of living process requires, for its success, tools which directly give people the language of form, the shapes with which to work. It requires that the people shaping things, planning, making the buildings, can, through the sequences of form language, share some coherence of style and geometric substance. That is what is needed, for people to have the competence that they need in order to make complete buildings, geometrically.
If there is to be a living structure in our society, the people of our society need such a form language. And I shall argue that it is our own responsibility (as architects) to create such a form language, both for our own use, and for the millions of people who are involved in making buildings.
But of course, unlike any one particular historic culture, which was able to embody its necessities in a particular language of forms that could be repeated, what we need for our time, and for all future time, is an understanding deep enough and general enough to include the coherence and the geometric unity of these past examples, while yet being free, open, flexible — able to take many individual forms, according to place, culture, and climate, and able, too, to change and evolve with changing history and changing technology.

#book/The Nature of Order/2 The process of creating life/16 Form language and style#