Syntax and informal semantic

Concrete syntax of the extension

Pendulum's keywords are based on Esterel's keywords

sname := <ocaml-expr-ident>
label := <ocaml-expr-ident>

sstatement :=
  | nothing
  | pause
  | emit <sname> <ocaml-expr>
  | present <presence_test_expr> <statement> <statement>
  | loop <statement>
  | <statement> || <statement>
  | <statement> ; <statement>
  | signal <sname> <ocaml-expr> <statement>
  | trap <label> <statement>
  | exit <label>
  | suspend <presence_test_expr> <statement>
  | (atom | !) <ocaml-expr>
  | present <presence_test_expr> <statement> (* not kernel *)

inout_decls :=
  | input sname `;` 
  | input sname `;` inout_decls

presence_test_expr :=
  | <sname> [& <ocaml-expr>]
  | <sname>##<sname> [& <ocaml-expr>]

program := inout_decls statement

Statements informal semantic

nothing

Does nothing.

---

pause

Stops and wait for the next instant.

---

emit <sname> <ocaml-expr>

Sets the signal <sname> present for this instant in the environnement and assigns it the given OCaml value

---

present <presence_test_expr> <statement> <statement>

Runs the first <statement> if the signal <sname> otherwise.

---

loop <statement>

Re-runs <statement> when it stops for ever.

---

<statement> || <statement>

Runs the two statements in parallele. Ends when the both end.

---

<statement> ; <statement>

Runs the first <statement> and then the second at the same instant the first ends.

---

signal <sname> <ocaml-expr> <statement>

Defines sname as a local signal with the given initial value in the <statement>

---

trap <label> <statement>

Jump at the end if <statement> runs exit <label>

---

exit <label>

See previous entry.

---

suspend <presence_test_expr> <statement>

Pauses <statement> each instants <sname> is present.

---

atom <ocaml-expr> or !<ocaml-expr>

Runs the given instantaneous OCaml expression (seen as a side effet).

---

present <presence_test_expr> <statement>

Same as present <sname> <statement> nothing

Presence test expressions

presence_test_expr :=
  | <sname> [& <ocaml-expr>]
  | <sname>##<sname> [& <ocaml-expr>]

You can add a guard to a presence test with the operator &

Syntax's precedence rules

The syntax's precedence rules follow those of OCaml corresponding expressions :

• keywords have the function call precedence. example :

loop
  atom
    print_string "hello";
  pause

is incorrect. The statement inside loop must be parenthezised with begin...end or (...) and so does the atom

loop begin
  atom (print_string "hello");
  pause
end 

• || and ; has the same precedence than in OCaml, particularly :
• Both are weaker than function calls so loop pause || loop pause and loop pause ; loop pause are syntactically corrects and do what you expect
• ; is weaker than || so :

loop
  pause
|| 
atom (print_string "hello");
pause

is equivalent to

(
  (loop pause)
  ||
  (atom (print_string "hello"))
)
;
(
  pause
)