Fan’s plugins

• A native example

  Suppose we want a new syntax for expressions,

  (matches <pattern> ) or (matches when <pattern> when <expr>))
  

  that desugars into a test function returning a boolean:

  (function | <pattern> when <expr> -> true | _ -> false)
  

  Then we could create a file as follows:

  %create{ p};;
  
  %extend{
    p:
    [ pat{p};"when"; exp{e} ->
       %exp{ function | $p when $e -> true |_ -> false }
    | pat{p} -> %exp{ function | $p -> true | _ -> false } ]
  };;
  
  let d = `Absolute["Tutorial"];;
  AstQuotation.of_exp ~name:(d,"q") ~entry:p;;
  

  1. Create a new entry p for this DDSL
  2. Write the grammar for the new DDSL
  3. Register the new DDSL named q, in the namespace Tutorial.

To compile the file:

$ ocamlc -pp 'fan.native' testq.ml

• Test on the fly

  We can eval the code on the fly without installing the plugins using the preprocessor fane.byte

  As follows:

  #{:eval|
  open  Fsyntax
  {:create| Fgram p|};;
  
  {:extend|
    p:
    [pat{p};"when"; exp{e} -> {:exp| function | $pat:p when $e -> true
    |_ -> false |}
    |pat{p} -> {:exp| function | $pat:p -> true | _ -> false |} ]
  |};;
  let d = `Absolute["Tutorial"];;
  AstQuotation.of_exp ~name:(d,"q") ~entry:p;;
  |}
  
  
  #{:control| import Tutorial ; |}
  if {:q| {:exp-| $a+$b|}|} {:exp-| 3 + 4|} then
      print_endline "yes!"
  else print_endline "no"
  

  To compile the file, you only need to switch from the preprocessor fan.byte to fane.byte:

  $ ocamlc -pp 'fane.byte ' langQ.ml && ./a.out