Caching • styler

library(styler)

This is a developer vignette to explain how caching works and what we learned on the way.

The main caching features were implemented in the following two pull requests:

#538: Implemented simple caching and utilities for managing caches. Input text is styled as a whole and added to the cache afterwards. This makes most sense given that the very same expression will probably never be passed to styler, unless it is already compliant with the style guide. Apart from the (negligible) inode, caching text has a memory cost of 0. Speed boosts only result if the whole text passed to styler is compliant to the style guide in use. Changing one line in a file with hundreds of lines means each line will be styled again. This is a major drawback and makes the cache only useful for a use with a pre-commit framework (the initial motivation) or when functions like style_pkg() are run often and most files were not changed.
#578: Adds a second layer of caching by caching top-level expressions individually. This will bring speed boosts to the situation where very little is changed but there are many top-level expressions. Hence, changing one line in a big file will invalidate the cache for the expression the line is part of, i.e. when changing x <- 2 to x = 2 below, styler will have to restyle the function definition, but not another(call) and all other expressions that were not changed.

function() {
  # a comment
  x <- 2 # <- change this line
}

another(call)

While #538 also required a lot of thought, this is not necessarily visible in the diff. The main challenge was to figure out how the caching should work conceptually and where we best insert the functionality as well as how to make caching work for edge cases like trailing blank lines etc. For details on the conceptual side and requirements, see #538.

In comparison, the diff in #578 is much larger. We can walk through the main changes introduced here:

Each nest gained a column is_cached to indicate if an expression is cached. It’s only ever set for the top-level nest, but set to NA for all other nests. Also, comments are not cached because they are essentially top level terminals which are very cheap to style (also because hardly any rule concerns them) and because each comment is a top-level expression, simply styling them is cheaper than checking for each of them if it is in the cache.
Each nest also gained a column block to denote the block to which it belongs for styling. Running each top-level expression through parse_transform_serialize_r() separately is relatively expensive. We prefer to put multiple top-level expressions into a block and process the block. This is done with parse_transform_serialize_r_block(). Note that before we implemented this PR, all top-level expressions were sent through parse_transform_serialize_r() as one block. Leaving out some exceptions in this explanation, we always put uncached top-level expressions in a block and cached top-level expressions into a block and then style the uncached ones.
Apart from the actual styling, a very costly part of formatting code with styler is to compute the nested parse data with compute_parse_data_nested(). When caching top-level expressions, it is evident that building up the nested structure for cached code is unnecessary because we don’t actually style it, but simply return text. For this reason, we introduce the concept of a shallow nest. It can only occur at the top level. For the top-level expressions we know that they are cached, we remove all children before building up the nested parse table and let them act as terminals and will later simply return their text. Hence, in the nested parse table, no cached expressions have children.
Because we now style blocks of expressions and we want to preserve the line breaks between them, we need to keep track of all blank lines between expressions, which was not necessary previously because all expressions were in a block and the blank lines separating them were stored in newlines and lag_newlines except for all blank lines before the first expression.
Because we wanted to cache by expression, but process by block of expression, we needed to decompose the block into individual expressions and add them to the cache once we obtained the final text. We could probably also have added expressions to the cache before we put the text together, but the problem is that at some point we turn the nested structure into a flat structure and as this must happen with a post_visit() approach, we’d have to implement a complicated routine to check if we are now about to put together all top-level expressions and then if yes write them to the cache. A simple (but maybe not so elegant) parsing of the output as implemented in cache_by_expression() seemed reasonable in terms of limiting complexity and keeping efficiency.

For more detailed explanation and documentation, please consult the help files of the internals.