register_transformation name registers a code transformation.
name is a logical name for the transformation (such as sexp_conv or bin_prot). It is mostly used for debugging purposes.
rules is a list of context independent rewriting rules, such as extension point expanders. This is what most code transformation should use. Rules from all registered transformations are all applied at the same time, before any other transformations. Moreover they are applied in a top-down manner, giving more control to extensions on how they interpret their payload.
For instance:
- some extensions capture a pretty-print of the payload in their expansion and using top-down ensures that the payload is as close as possible to the original code
- some extensions process other extension in a special way inside their payload. For instance
%here (from ppx_here) will normally expand to a record of type Lexing.position. However when used inside %sexp (from ppx_sexp_value) it will expand to the human-readable sexp representation of a source code position.
extensions is a special cases of rules and is deprecated. It is only kept for backward compatibility.
enclose_impl and enclose_intf produces a header and footer for implementation/interface files. They are a special case of impl and intf. Both functions receive a location that denotes the whole file, or None if the file contains nothing.
impl is an optional function that is applied on implementation files and intf is an optional function that is applied on interface files. These two functions are applied on the AST of the whole file. They should only be used when the other mechanism are not enough. For instance if the transformation expands extension points that depend on the context.
If no rewriter is using impl and intf, then the whole transformation is completely independent of the order in which the various rewriter are specified. Moreover the resulting driver will be faster as it will do only one pass (excluding safety checks) on the whole AST.