Braces are no longer a type: (Map K V) is the only spelling

The author's decision, and it removes the one syntax question generics
had. A return type can no longer be written in braces, so a {...} after
the signature is unambiguously the constraint map and there is no
structural rule to explain.

The reasons for the record: the brace's value meaning and its type
meaning do not correspond the way the bracket's do - [1 2 3] is a value
whose type is [3 i32], but {.x 1} is a value whose type is a name, and a
map value is built by map-new with no braces anywhere - and dropping it
reserves {} in type position for anonymous struct types.

Braces in a type are refused with the surviving spelling named rather
than falling through to "expected a type". Types.to_string and
Cimport's source printer both print (Map K V) now, and Shim refuses the
application spelling where it used to refuse only Ast.Tmap.

lib/cimport.ml

@@ -392,7 +392,8 @@ let rec ty_source (t : Ast.texpr) =
   | Ast.Tslice e -> Printf.sprintf "[%s]" (ty_source e)
   | Ast.Tarray (Ast.Lint n, e) -> Printf.sprintf "[%Ld %s]" n (ty_source e)
   | Ast.Tarray (Ast.Lname n, e) -> Printf.sprintf "[%s %s]" n (ty_source e)
-  | Ast.Tmap (k, v) -> Printf.sprintf "{%s %s}" (ty_source k) (ty_source v)
+  | Ast.Tmap (k, v) ->
+    Printf.sprintf "(Map %s %s)" (ty_source k) (ty_source v)
   | Ast.Tfn (ps, r) ->
     Printf.sprintf "(Fn [%s] %s)"
       (String.concat " " (List.map ty_source ps)) (ty_source r)

lib/parse.ml

@@ -65,8 +65,26 @@ let rec texpr (f : Form.t) : Ast.texpr =
   | Vec [ n; elem ] -> mk (Ast.Tarray (len n, texpr elem))
   | Vec _ ->
     fail f "a type in brackets is [T] for a slice or [n T] for a fixed array"
-  | Map [ k; v ] -> mk (Ast.Tmap (texpr k, texpr v))
-  | Map _ -> fail f "a map type is {K V}"
+  (* Braces are not a type. [{K V}] used to spell [(Map K V)] and the two
+     resolved to the same thing; the brace spelling is withdrawn, and the
+     refusal names the surviving one rather than letting the form fall through
+     to "expected a type".
+
+     Two reasons, and the second is the one that decided it. The brace's value
+     meaning and its type meaning do not correspond the way the bracket's do:
+     [[1 2 3]] is a value whose type is [[3 i32]], but [{.x 1 .y 0}] is a
+     value whose type is a *name*, and a map value is built by [map-new] with
+     no braces anywhere. And dropping it reserves [{}] in type position for
+     anonymous struct types, [{.x f32 .y f32}], which is a likelier thing to
+     want than a second spelling of a type that already has one.
+
+     It also settles the one syntax question generics had: a defn's constraint
+     map, [{:where (ordered? $t)}], sits immediately after the return type,
+     and with braces gone from type position there is nothing for it to be
+     confused with. *)
+  | Map _ ->
+    fail f "a map type is written (Map K V), not in braces — braces in type \
+            position are not a type" 
   | List ({ v = Sym "Fn"; _ } :: rest) ->
     (match rest with
      | [ { v = Vec params; _ }; ret ] ->
@@ -99,21 +117,18 @@ let rec fields (f : Form.t) (items : Form.t list) : Ast.field list =
    rather than a bare keyword: it leaves room for further keys without new
    syntax.
 
-   **The disambiguation, since it is the one syntax question the feature had
-   to settle.** [{K V}] is a legal *return type* spelling for [(Map K V)], so
-   [(defn f [xs [$t]] {string i32} {:where ...} body)] puts two braces in a
-   row meaning different things. They are told apart structurally, by the
-   first form inside: a constraint map leads with a *keyword*, and a map type
-   leads with a type — [{string i32}], [{K V}] — and a keyword is not a type
-   anywhere in the language. So [Map ({v = Kw _} :: _)] in the slot after the
-   return type is a constraint map and nothing else can be. The return-type
-   slot itself is never ambiguous: [Parse] takes it unconditionally, before
-   this is consulted. [{K V}] stays exactly as it was — whether it survives is
-   a separate open question, and this feature does not force it.
+   **The one syntax question it had, and how it stopped being one.** [{K V}]
+   used to be a legal *return type* spelling for [(Map K V)], which put two
+   braces in a row meaning different things — [(defn f [xs [$t]] {string i32}
+   {:where ...} body)]. The brace spelling has since been withdrawn from type
+   position entirely ([texpr] above), so the slot after the return type can be
+   nothing but this. A bare [{}] in *expression* position is already refused
+   ([expr] below), so there is nothing for it to be confused with on the other
+   side either.
 
-   A bare [{}] in *expression* position is already refused ([expr] below), so
-   there is also nothing for a constraint map to be confused with once past
-   the return type. *)
+   The leading keyword is still required and still checked, because it is what
+   tells a constraint map from a struct literal's field list, [{.x 1}], which
+   is what braces mean in the position a body starts in. *)
 let constraints (body : Form.t list) : Ast.pred list * Form.t list =
   match body with
   | ({ Form.v = Form.Map (({ Form.v = Form.Kw _; _ } :: _ as kvs)); _ } as m)

lib/shim.ml

@@ -222,7 +222,11 @@ let rec cty env ~needed ~loc ~what (t : Ast.texpr) : string =
       "%s is a fixed array, which C passes as a pointer and Flan as a value — \
        declare (Ptr T) and say which"
       what
-  | Ast.Tmap _ -> fail loc "%s is a map, which has no C representation" what
+  (* Two spellings reach the same type: [Ast.Tmap], which only [Cimport]
+     builds now, and [(Map K V)], which is what source writes since the brace
+     spelling was withdrawn from type position. Both are refused here. *)
+  | Ast.Tmap _ | Ast.Tapp ("Map", _) ->
+    fail loc "%s is a map, which has no C representation" what
   (* A Vec owns its storage, so handing its header to C hands out an owner and
      there is no rule for what C would then be allowed to do with it. The
      elements cross the way any other run of elements does. *)

lib/types.ml

@@ -133,7 +133,7 @@ let rec to_string = function
   | Named n | Enum n -> n
   | Slice t -> "[" ^ to_string t ^ "]"
   | Array (n, t) -> Printf.sprintf "[%Ld %s]" n (to_string t)
-  | Map (k, v) -> Printf.sprintf "{%s %s}" (to_string k) (to_string v)
+  | Map (k, v) -> Printf.sprintf "(Map %s %s)" (to_string k) (to_string v)
   | Ptr t -> "(Ptr " ^ to_string t ^ ")"
   | Alloc -> "Allocator"
   | Vec t -> "(Vec " ^ to_string t ^ ")"

syntax-sketch.flan

@@ -18,13 +18,17 @@
 ;;   [4 f32]       fixed array — a value, copies on assignment
 ;;   [f32]         slice, ptr+len — a NON-OWNING view, copies the view only
 ;;   (Vec f32)     owning growable, ptr+len+cap — MOVE-ONLY, carries allocator
-;;   {string i32}  owning hashmap — move-only, shorthand for (Map string i32)
+;;   (Map string i32)  owning hashmap — move-only
 ;;
-;; Braces are read by position: in a TYPE position {K V} is a map type; in a
-;; VALUE position {.field v ...} is a struct or condition literal — a field
-;; label is a dot, and the colon is left for keys. There is no map literal yet;
-;; a map is built with make-map and an allocator, and when a literal arrives it
-;; takes {:key value}, which is why the dot is what struct construction uses.
+;; Braces are NOT a type. {K V} used to be a second spelling of (Map K V) and
+;; was withdrawn: the brace's value and type meanings do not correspond the way
+;; the bracket's do, and {} in type position is wanted for anonymous struct
+;; types, {.x f32 .y f32}. In a VALUE position {.field v ...} is a struct or
+;; condition literal — a field label is a dot, and the colon is left for keys.
+;; There is no map literal yet; a map is built with map-new and an allocator,
+;; and when a literal arrives it takes {:key value}, which is why the dot is
+;; what struct construction uses. A defn's constraint map, {:where (ordered?
+;; $t)}, is the other brace form, and it sits after the return type.
 ;;   (Ptr World)   pointer
 ;;   (Fn [f32] bool)  function pointer, no captured environment
 ;;   (Option a)    union from the stdlib

test/test_acceptance.ml

@@ -1844,7 +1844,7 @@ ERR@7 unexpected token: not the kind the caller was reading
       "(declare-c takes [xs [4 f32]] \"Takes\")"
       "which C passes as a pointer and Flan as a value";
     shim_refuses "declare-c: a map"
-      "(declare-c takes [m {string i32}] \"Takes\")"
+      "(declare-c takes [m (Map string i32)] \"Takes\")"
       "which has no C representation";
     shim_refuses "declare-c: a returned string"
       "(declare-c name [] string \"Name\")"

test/test_flan.ml

@@ -412,8 +412,10 @@ let () =
    | _ -> check "nested array with named lengths" false);
   (match ty "(Ptr Cursor)" with
    | Tapp ("Ptr", [ _ ]) -> () | _ -> check "(Ptr T)" false);
-  (match ty "{string i32}" with
-   | Tmap (_, _) -> () | _ -> check "{K V} is a map type" false);
+  (* A map type is an application like (Ptr T) and (Vec T) now that the brace
+     spelling is gone: [Ast.Tmap] survives only as what [Cimport] builds. *)
+  (match ty "(Map string i32)" with
+   | Tapp ("Map", [ _; _ ]) -> () | _ -> check "(Map K V) is a map type" false);
   (match ty "(Fn [a a] bool)" with
    | Tfn ([ _; _ ], _) -> () | _ -> check "(Fn [T] R)" false);
 
@@ -858,11 +860,11 @@ let () =
      back as generics. *)
   rejects_check "Vec takes one type"    "(defn f [x (Vec i32 i32)] ())"
     ~needle:"exactly one type";
-  (* {K V} resolves now — it is the Map type spelling, and the only one, since
-     a bare map form in expression position is a struct literal's field list.
-     What is still refused is the arity, for the same reason Vec's is: a
-     near-miss would otherwise resolve to a type variable and come back as
-     generics. *)
+  (* (Map K V) is the map type spelling, and now the only one: the brace form
+     is withdrawn from type position, so braces there are refused with the
+     surviving spelling named. What is refused here is the arity, for the same
+     reason Vec's is: a near-miss would otherwise resolve to a type variable
+     and come back as generics. *)
   rejects_check "Map takes two types"   "(defn f [x (Map i32)] ())"
     ~needle:"exactly two types";
   rejects_check "Result is milestone 6" "(defn f [] (Result i32 i32) None)"
@@ -2118,21 +2120,22 @@ let () =
       | [] -> check "a report has a first line" false)
    | None -> check "a report needs a diagnostic" false);
 
-  (* ── Generics: the syntax, the predicates, and the two defaults ──
-     The syntax question the feature had to settle first: [{K V}] is a legal
-     *return type*, so a defn with a map return type and a constraint map puts
-     two braces in a row meaning different things. They are told apart
-     structurally, by the first form inside — a constraint map leads with a
-     keyword and a map type leads with a type — so [{K V}] did not have to go
-     and is still exactly what it was. *)
-  accepts "a map return type is still a map return type"
-    "(defn f [] {string i32} (map-new string i32))";
+  (* ── Generics: the syntax, the predicates, and the two defaults ── *)
+
+  (* The one syntax question the feature had, and how it stopped being one.
+     [{K V}] used to be a legal *return type* spelling for (Map K V), so a
+     defn with a map return type and a constraint map put two braces in a row
+     meaning different things. The brace spelling is now withdrawn from type
+     position entirely, so the slot after the return type can be nothing but
+     the constraint map, and braces in a type say where the spelling went. *)
+  accepts "a map return type, written the one way there is"
+    "(defn f [] (Map string i32) (map-new string i32))";
   accepts "a map return type followed by a constraint map"
-    "(defn f [x $t] {string i32} {:where (copyable? $t)} \
+    "(defn f [x $t] (Map string i32) {:where (copyable? $t)} \
      (do x (map-new string i32)))";
-  rejects_check "a map return type is not read as a constraint map"
-    ~needle:"is not a type variable of f"
-    "(defn f [] {string i32} {:where (ordered? $t)} (map-new string i32))";
+  rejects_check "braces in type position say where the spelling went"
+    ~needle:"written (Map K V)"
+    "(defn f [] {string i32} (map-new string i32))";
 
   (* The predicates, and each one gating the operator it is for. *)
   accepts "ordered? admits <"
@@ -2194,9 +2197,9 @@ let () =
      they are chosen from the concrete type, which does not exist yet. *)
   rejects_check "a map keyed by a type variable that is not hashable?"
     ~needle:"is not a map key"
-    "(defn f [m {$t i32}] i32 {:where (copyable? $t)} (len m))";
+    "(defn f [m (Map $t i32)] i32 {:where (copyable? $t)} (len m))";
   accepts "and hashable? is what says it is"
-    "(defn f [m {$t i32}] i32 {:where (hashable? $t)} (len m))";
+    "(defn f [m (Map $t i32)] i32 {:where (hashable? $t)} (len m))";
 
   (* ── The acceptance program checks end to end ──────────────────── *)
   accepts "calc-me.flan type checks"