package metaquot

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OCaml syntax extension for quoting code


Dune Dependency






metaquot allows to quote OCaml code.

Published: 07 Oct 2021


metaquot: OCaml syntax extension for quoting code

metaquot is a PPX rewriter that provides several extensions for quoting OCaml code in expressions, à la ppxtools.ppx_metaquot and ppxlib.metaquot. In comparison to these libraries, metaquot is built by meta-programmation over the Parsetree module (thanks to metapp) and is meant to be trivial to update for future versions of OCaml (on the other hand, metaquot only builds AST for the current version of OCaml: you may use helpers provided by Ast_helper or Metapp to manipulate the AST in a version-independent manner).

metaquot can be used with dune by using the preprocess field.

  (preprocess (pps metaquot.ppx))

The following extensions are provided.

Quotation Type
[%expr ...] Parsetree.expression
[%pat? ...] Parsetree.pattern
[%type: ...] Parsetree.core_type
[%sig: ...] Parsetree.signature
[%sigi: ...] Parsetree.signature_item
[%str ...] Parsetree.structure
[%stri ...] Parsetree.structure_item
[%lid ...] Parsetree.longident

The produced AST uses !Ast_helper.default_loc as location: you may change the location with Ast_helper.with_default_loc.

Moreover, in quoted code, the following extensions allow to evaluate and insert expressions or patterns (anti-quotations).

Anti-quotation Type
[%e ...] Parsetree.expression
[%p ...] Parsetree.pattern
[%t ...] Parsetree.core_type
[%m ...] Parsetree.module_type or Parsetree.module_expr
[%i ...] Parsetree.signature_item or Parsetree.structure_item

To insert expressions or patterns to syntactic places where anti-quotations are not supported, substitutions can be specified in attributes. For instance:

  • [%str module M = struct ... end][@subst let M : string = s] defines a module, the name of which is given by an expression s of type string

  • [%str match x with c -> .][@subst let c : list = cases] constructs a pattern matching, the cases of which is given by an expression cases of type case list (note that the substitution is specified by giving only the type name list).

More generally, substitutions are given using let-binding syntax, specifying the name of the substituted identifier, the name of the type of the AST node to be replaced, and the expression or the pattern to use for the substitution: when the identifier appears, the closest parent AST node of the given type name is replaced by the given expression or pattern. Currently, only binding variable names are supported for patterns. The [@subst] attribute can be carried either by the quotation extension itself or by any subelement.

Inside quoted expressions, the [@for] attribute can be used to repeat an element in an AST node of type _ list. For instance, [%str type t1 and t2 and t3] can be written [%str type t [@@subst let t : string = name][@@for name := ["t1"; "t2"; "t3"]]]. The element is repeated in the closest parent AST node of type _ list.

In addition to the syntax extension, the metaquot package provides the Metaquot module, which contains lifters: the Metaquot.Exp module lifts to expressions and Metaquot.Pat lifts to patterns. For instance, Metaquot.Exp.pattern : Parsetree.pattern -> Parsetree.expression is a function that returns an OCaml expression that builds the AST corresponding to the given pattern.

Dependencies (6)

  1. metapp >= "0.4.1" & < "0.4.4"
  2. dune >= "1.11.0"
  3. ocamlfind >= "1.8.1"
  4. ppxlib >= "0.22.0"
  5. stdcompat >= "12"
  6. ocaml >= "4.08.0" & < "4.14"

Dev Dependencies (1)

  1. odoc with-doc & >= "1.5.1"

Used by (6)

  1. clangml >= "4.3.0"
  2. override >= "0.4.0"
  3. pattern >= "0.3.0"
  4. ppx_pbt < "0.2.1"
  5. refl >= "0.4.0"
  6. traverse >= "0.3.0"

Conflicts (1)

  1. ocaml-variants = "4.12.0+domains"

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