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title: "Caching"
output: rmarkdown::html_vignette
vignette: >
```{r, include = FALSE}
collapse = TRUE,
comment = "#>"
```{r setup}
This is a developer vignette to explain how caching works and what we learned on
the way. To use the caching feature, please have a look at the README.
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) innode, 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.
```{r, eval = FALSE}
function() {
# a comment
x = 2 # <- change this line
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.
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