flan/lib/render.ml

(** The structural printer: a compile-time walk over a [Tast] type that emits
    the calls which print a value of it.

    It lives apart from its two callers because there are two, and they differ
    in exactly one thing: where the pieces go. The REPL sends them to
    [flan_dev_emit] ([session.ml]); [println] sends them to stdout
    ([check.ml]). Everything else — which arm a type takes, how an enum
    recovers its member names, the depth and span caps — has to be the same in
    both, and the way to make it the same is to have one copy.

    Why the walk is at compile time at all: a Flan value carries no header, so
    nothing at run time could say what it is. The compiler knows the type and
    renders it there. And why it emits piecewise rather than building a string:
    a struct is its fields with punctuation between them, and concatenating
    that would need an allocator the language does not have.

    The emitter is five functions rather than five names because the two sides
    are not both extern calls. The REPL's are ([flan_dev_emit_i64] takes an
    i64); stdout's compose a conversion with a write — [(write-stdout
    (i64->bytes x))] — and a name alone cannot say that. *)

(* Each takes a value of the type its field is named for and returns a Unit
   expression that prints it. [estr] is handed a [u8] slice and is expected to
   quote and escape it: it is the *nested* string case, the one inside a struct
   or an array, where an unquoted run of bytes could not be told from the
   punctuation around it. A caller that wants a string printed raw does not go
   through the walk at all. *)
type emitter = {
  ebytes : Tast.expr -> Tast.expr;   (* [u8], verbatim: punctuation and literals *)
  estr   : Tast.expr -> Tast.expr;   (* [u8], quoted and escaped *)
  ei64   : Tast.expr -> Tast.expr;
  eu64   : Tast.expr -> Tast.expr;
  ef64   : Tast.expr -> Tast.expr;
}

type ctx = {
  structs : Tast.structure list;
  enums   : (string * (string * int64) list) list;
  emit    : emitter;
  (* A slot in the *caller's* frame. Only the slice arm needs one, and it needs
     two: the slice itself, so the expression it came from is evaluated once
     rather than once per element, and the loop counter. Who owns the frame
     differs — the REPL's is a thunk it is building, [println]'s is the user
     function being checked — so allocating one is the caller's to do. *)
  alloc   : Types.t -> int;
}

(* Two separate limits, easily conflated. [depth] and [span] bound the *walk*,
   so a big fixed array or a self-containing struct cannot turn one expression
   into a module with ten thousand render sites in it. How much text actually
   comes out is bounded in the runtime instead, once, for every renderer. *)
let max_depth = 4
let max_span = 8

let fail = Loc.fail

let rec render c depth (e : Tast.expr) : Tast.expr list =
  let loc = e.Tast.loc in
  let unit_ e = { Tast.e; ty = Types.Unit; loc } in
  let cast t x = { Tast.e = Tast.Prim (Tast.Cast t, [ x ]); ty = t; loc } in
  let bytes_of s =
    { Tast.e = Tast.Prim (Tast.Bytes, [ { Tast.e = Tast.Str s; ty = Types.String; loc } ]);
      ty = Types.Slice (Types.Int Types.U8); loc }
  in
  let lit s = c.emit.ebytes (bytes_of s) in
  let int64 n = { Tast.e = Tast.Int (n, Types.I64); ty = Types.Int Types.I64; loc } in
  let i32 n =
    { Tast.e = Tast.Int (Int64.of_int n, Types.I32); ty = Types.Int Types.I32; loc }
  in
  let do_ xs = unit_ (Tast.Do xs) in
  if depth > max_depth then [ lit "..." ]
  else
    match e.Tast.ty with
    | Types.Int Types.U64 -> [ c.emit.eu64 e ]
    | Types.Int _ -> [ c.emit.ei64 (cast (Types.Int Types.I64) e) ]
    | Types.Float _ -> [ c.emit.ef64 (cast (Types.Float Types.F64) e) ]
    | Types.Bool ->
      [ unit_ (Tast.If (e, lit "true", lit "false")) ]
    (* Evaluated *and then* reported. A Unit expression is almost always a call
       made for its effect — (print-line "x") is the REPL's most ordinary
       input — so emitting the literal without running it would make the prompt
       answer () while nothing happened. *)
    | Types.Unit -> [ e; lit "()" ]
    | Types.String ->
      [ c.emit.estr
          { Tast.e = Tast.Prim (Tast.Bytes, [ e ]);
            ty = Types.Slice (Types.Int Types.U8); loc } ]
    (* Bytes are almost always text, and escaping makes the case where they are
       not readable rather than a mess. *)
    | Types.Slice (Types.Int Types.U8) -> [ c.emit.estr e ]
    (* An enum's members are erased to i32 before the backend sees them, so the
       name has to be recovered here, from the checker's table, as a chain of
       comparisons. Falling through to the number is not a failure: a value
       outside the declared members is exactly what you would want to see. *)
    | Types.Enum n ->
      let members = try List.assoc n c.enums with Not_found -> [] in
      let number = c.emit.ei64 (cast (Types.Int Types.I64) e) in
      List.fold_left
        (fun otherwise (name, v) ->
           let is =
             { Tast.e =
                 Tast.Prim (Tast.Eq,
                            [ cast (Types.Int Types.I64) e; int64 v ]);
               ty = Types.Bool; loc }
           in
           unit_ (Tast.If (is, lit (":" ^ name), otherwise)))
        number members
      |> fun x -> [ x ]
    (* A pointer is rendered as its shape and never followed: it is the only
       thing that could make this walk cycle, and dereferencing one a REPL was
       handed is not a safe thing to do on someone's behalf. *)
    | Types.Ptr _ -> [ lit "<ptr>" ]
    | Types.Option t ->
      let tag = { Tast.e = Tast.Field (e, 0); ty = Types.Int Types.I8; loc } in
      let some = { Tast.e = Tast.Field (e, 1); ty = t; loc } in
      let is_some =
        { Tast.e =
            Tast.Prim (Tast.Ne,
                       [ tag; { Tast.e = Tast.Int (0L, Types.I8);
                                ty = Types.Int Types.I8; loc } ]);
          ty = Types.Bool; loc }
      in
      [ unit_
          (Tast.If (is_some,
                    do_ ((lit "(some " :: render c (depth + 1) some) @ [ lit ")" ]),
                    lit "none")) ]
    | Types.Named n ->
      (match
         List.find_opt (fun (s : Tast.structure) -> String.equal s.Tast.sname n)
           c.structs
       with
       | None -> [ lit ("<" ^ n ^ ">") ]
       | Some st ->
         let fields = st.Tast.fields in
         let shown = List.filteri (fun i _ -> i < max_span) fields in
         let parts =
           List.concat
             (List.mapi
                (fun i (f : Tast.field) ->
                   let v = { Tast.e = Tast.Field (e, i); ty = f.Tast.fty; loc } in
                   (if i = 0 then [] else [ lit " " ])
                   @ [ lit (":" ^ f.Tast.fname ^ " ") ]
                   @ render c (depth + 1) v)
                shown)
         in
         [ do_ ((lit ("(" ^ n ^ " {") :: parts)
                @ (if List.length fields > max_span then [ lit " ..." ] else [])
                @ [ lit "})" ]) ])
    (* A fixed array's length is in its type, so it unrolls — capped, because
       sand's grid is [100 [100 u32]] and unrolling that is ten thousand render
       sites in one module. *)
    | Types.Array (n, t) ->
      let shown = min (Int64.to_int n) max_span in
      let parts =
        List.concat
          (List.init shown (fun i ->
             let v =
               { Tast.e = Tast.Prim (Tast.At, [ e; i32 i ]); ty = t; loc }
             in
             lit " " :: render c (depth + 1) v))
      in
      [ do_ ((lit "[" :: parts)
             @ (if Int64.to_int n > shown then [ lit " ..." ] else [])
             @ [ lit "]" ]) ]
    (* A slice's length is not known until it runs, so this is the one case
       that needs a loop. The slice goes into a slot first: the expression it
       came from must not be evaluated once per element. *)
    | Types.Slice t ->
      let sv = c.alloc e.Tast.ty and iv = c.alloc (Types.Int Types.I32) in
      let local i ty = { Tast.e = Tast.Local i; ty; loc } in
      let len =
        { Tast.e = Tast.Prim (Tast.Len, [ local sv e.Tast.ty ]);
          ty = Types.Int Types.I32; loc }
      in
      let cond =
        { Tast.e = Tast.Prim (Tast.Lt, [ local iv (Types.Int Types.I32); len ]);
          ty = Types.Bool; loc }
      in
      let elem =
        { Tast.e =
            Tast.Prim (Tast.At, [ local sv e.Tast.ty; local iv (Types.Int Types.I32) ]);
          ty = t; loc }
      in
      let step =
        unit_
          (Tast.Set
             (Tast.Plocal iv,
              { Tast.e =
                  Tast.Prim (Tast.Add, [ local iv (Types.Int Types.I32); i32 1 ]);
                ty = Types.Int Types.I32; loc }))
      in
      [ unit_
          (Tast.Let
             ([ (sv, e); (iv, i32 0) ],
              [ lit "[";
                unit_
                  (Tast.While
                     (cond, (lit " " :: render c (depth + 1) elem) @ [ step ]));
                lit "]" ])) ]
    | t ->
      fail loc "no printer for %s" (Types.to_string t)