Decoders in dune

M_A_1 · August 1, 2024, 5:14pm

I am currently working on some project that involves Ocaml’s build system Dune. However, in my work, there is some tasks that require taking a file (mainly its path) which contains some text written in a S-expression format. For example, for those familiar with defining and building rules with dune,

(rule (deps (file a) (glob *.cmi)) (action cat a))

By investigating the dune codebase, I have understood that there is a decoder (which is actually a monad) that is defined for a type 'a, so that when this decoder is a ran on a certain S-expression it returns the object of type 'a defined in this S-expression (if it is a valid one).

My question is how exactly one could use such a defined decoder on custom file (that is not a dune file that is automatically parsed by dune) in order to extract the information? I assume that one should first parse the file in order to extract the S-expression, but I don’t know if that is automatically done by the decoder.Preformatted text

nojb · August 1, 2024, 10:51pm

Broadly speaking, the file can be parsed using the module Parser:

github.com

ocaml/dune/blob/0a8af6e1c8eb701e43cefa92bcfd131ec89ed6d7/src/dune_sexp/parser.mli#L15


      
          module Mode : sig
            type 'a t =
              | Single : Ast.t t
              | Many : Ast.t list t
              | Many_as_one : Ast.t t
              | Cst : Cst.t list t
          end
          
          val parse : mode:'a Mode.t -> ?lexer:Lexer.t -> Lexing.lexbuf -> 'a
          val parse_string : fname:string -> mode:'a Mode.t -> ?lexer:Lexer.t -> string -> 'a
          val load : ?lexer:Lexer.t -> Path.t -> mode:'a Mode.t -> 'a
          
          (** Insert comments in a concrete syntax tree. Comments are inserted based on
              their location. *)
          val insert_comments : Cst.t list -> (Loc.t * string list) list -> Cst.t list

This will yield an Ast.t or Ast.t list (assuming you pass Single or Many for the ~mode argument). Then a decoder can be run over this value by calling the only function available with the required signature:

github.com

ocaml/dune/blob/0a8af6e1c8eb701e43cefa92bcfd131ec89ed6d7/src/dune_sexp/decoder.mli#L35


      
          type ('a, 'kind) parser
          
          type values
          type fields
          type 'a t = ('a, values) parser
          type 'a fields_parser = ('a, fields) parser
          
          (** [parse parser context sexp] parse a S-expression using the following parser.
              The input consist of a single S-expression. [context] allows to pass extra
              information such as versions to individual parsers. *)
          val parse : 'a t -> Univ_map.t -> Ast.t -> 'a
          
          (** [return] is a parser that returns the given value. *)
          val return : 'a -> ('a, _) parser
          
          (** [map t ~f] parses the input using [t] and then applies [f] to the result. *)
          val map : ('a, 'k) parser -> f:('a -> 'b) -> ('b, 'k) parser
          
          (** [t >>= f] parses the input using [t] and then uses [f] to parse the rest of
              the input. *)
          val ( >>= ) : ('a, 'k) parser -> ('a -> ('b, 'k) parser) -> ('b, 'k) parser

Cheers,
Nicolas

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Decoders in dune

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