vec-new and map-new take a type expression where they take a type, so (vec-new [u8]) makes a Vec of byte slices

lib/ast.ml

@@ -97,6 +97,12 @@ and expr_kind =
      fails on an unknown name. This is that position's answer, and it says what
      it does rather than looking like a vector of two things. *)
   | ArrayOf of texpr                  (* the whole array type, built by Parse *)
+  (* (vec-new [u8]) and (map-new string [u8]) — a type written where an
+     argument goes. Only the type positions of those two forms read one, and
+     only when the form's shape says type and not value: brackets, or a
+     parenthesised Ptr, Option, Vec, Map, Fn or CFn. A bare name stays a
+     [Var], which the checker already answers as a type. *)
+  | TypeArg of texpr
   (* (array-fill [r c] v) and (array-gen [r c] f) — a fixed array of any rank
      as an *expression*, which is what [ArrayOf] and [dotimes] between them
      could not be: [ArrayOf] produces the zeroed value only, and [dotimes] is
@@ -403,7 +409,7 @@ let map_children f (e : expr) : expr =
   let kind =
     match e.e with
     | Int _ | Float _ | Byte _ | Str _ | Kw _ | Quote _ | Var _ | ArrayOf _
-    | Break _ | Continue _ -> e.e
+    | TypeArg _ | Break _ | Continue _ -> e.e
     | Do es -> Do (List.map ex es)
     | Let (bs, es) -> Let (List.map bind bs, List.map ex es)
     | If (c, a, b) -> If (ex c, ex a, Option.map ex b)

lib/check.ml

@@ -3547,6 +3547,11 @@ let rec check ctx ?want (e : Ast.expr) : Tast.expr =
   | Ast.ArrayOf t ->
     let ty = resolve ctx.env t in
     expect ctx loc ~want (mk loc ty (Tast.Zero ty))
+  (* Parse writes one only into a type position of vec-new or map-new, and
+     those read it before it could get here. *)
+  | Ast.TypeArg _ ->
+    fail loc "internal: a type argument reached the checker outside vec-new or \
+              map-new — this is a compiler bug"
   | Ast.ArrayFill (dims, v) -> check_array_fill ctx ~want loc dims v
   | Ast.ArrayGen (dims, f) -> check_array_gen ctx ~want loc dims f
   | Ast.Match (scrutinee, arms) -> check_match ctx ~tail ?want loc scrutinee arms
@@ -6564,12 +6569,15 @@ and type_named ctx n =
   || Hashtbl.mem ctx.env.enums n
   || Hashtbl.mem ctx.env.aliases n
 
-(* The element type for [vec-new]: a leading bare symbol naming a type, or the
-   expectation at the site. A bare symbol shadowed by a local or a global is
-   that binding — an allocator, in practice — and not a type. *)
+(* The element type for [vec-new]: a leading bare symbol naming a type, a
+   leading type expression — [(vec-new [u8])], [(vec-new (Ptr Cell))], which
+   Parse has already read as one — or the expectation at the site. A bare
+   symbol shadowed by a local or a global is that binding — an allocator, in
+   practice — and not a type. *)
 and vec_new_elem ctx ~want loc args =
   let named =
     match args with
+    | { Ast.e = Ast.TypeArg t; _ } :: rest -> Some (resolve ctx.env t, rest)
     | { Ast.e = Ast.Var n; _ } :: rest
       when lookup ctx n = None
         && (not (Hashtbl.mem ctx.env.globals n))
@@ -6604,10 +6612,21 @@ and map_new_types ctx ~want loc args =
     && (not (Hashtbl.mem ctx.env.globals n))
     && type_named ctx n
   in
+  (* A type position holds a bare name or a type expression Parse has read
+     as one, as [vec-new]'s does. *)
+  let as_type (a : Ast.expr) =
+    match a.Ast.e with
+    | Ast.TypeArg t -> Some (resolve ctx.env t)
+    | Ast.Var n when is_type n -> Some (resolve_name ctx.env ~seen:[] loc n)
+    | _ -> None
+  in
   match args with
-  | { Ast.e = Ast.Var k; _ } :: { Ast.e = Ast.Var v; _ } :: rest
-    when is_type k && is_type v ->
-    resolve_name ctx.env ~seen:[] loc k, resolve_name ctx.env ~seen:[] loc v, rest
+  | k :: v :: rest when as_type k <> None && as_type v <> None ->
+    Option.get (as_type k), Option.get (as_type v), rest
+  | { Ast.e = Ast.TypeArg _; _ } :: _ ->
+    fail loc
+      "(map-new) names a key and no value — write both, as (map-new string \
+       i32), or give the binding a type"
   | { Ast.e = Ast.Var k; _ } :: rest when is_type k && rest = [] ->
     fail loc
       "(map-new %s) names a key and no value — write both, as (map-new %s \

lib/load.ml

@@ -307,6 +307,7 @@ let rec rename_expr owned alias bound (e : Ast.expr) : Ast.expr =
       Ast.MapLit (tag, List.map (fun (k, v) -> (go k, go v)) kvs)
     | Ast.Arr items -> Ast.Arr (gos items)
     | Ast.ArrayOf t -> Ast.ArrayOf (rename_texpr owned alias t)
+    | Ast.TypeArg t -> Ast.TypeArg (rename_texpr owned alias t)
     (* The dimensions too, for the reason [rename_texpr] gives about the one
        inside [Tarray]: a dimension written as a name is an ordinary
        compile-time constant of the package and has to be qualified like any
@@ -787,7 +788,7 @@ let rec expr_uses acc (e : Ast.expr) =
   | Ast.Bare kvs -> List.iter (fun (_, v) -> go v) kvs
   | Ast.MapLit (_, kvs) -> List.iter (fun (k, v) -> go k; go v) kvs
   | Ast.Arr items -> gos items
-  | Ast.ArrayOf t -> texpr_uses acc t
+  | Ast.ArrayOf t | Ast.TypeArg t -> texpr_uses acc t
   (* A dimension written as a name is a use of that constant, exactly as it is
      inside [Tarray]. *)
   | Ast.ArrayFill (ds, v) | Ast.ArrayGen (ds, v) ->

lib/parse.ml

@@ -425,6 +425,33 @@ and form f mk (head : Form.t) (args : Form.t list) : Ast.expr =
      | [ target; value ] -> mk (Ast.Set (place target, expr value))
      | _ -> fail f "set is (set place value)")
 
+  (* ── (vec-new [u8]) and (map-new string [u8]) ───────────────────────
+     The type positions of these two take a type expression as well as a bare
+     name. A bare name is left for the checker, which knows whether it names a
+     type or an allocator; a bracket or a parenthesised type constructor can
+     only be a type there, so it is read as one now, with [texpr], the reader
+     a parameter list's types go through. *)
+  | Sym (("vec-new" | "builtin/vec-new" | "map-new" | "builtin/map-new") as n) ->
+    let slots =
+      if n = "vec-new" || n = "builtin/vec-new" then 1 else 2
+    in
+    let is_type (a : Form.t) =
+      match a.v with
+      | Vec _ -> true
+      | List ({ v = Sym ("Ptr" | "Option" | "Vec" | "Map" | "Fn" | "CFn"); _ }
+              :: _ :: _) -> true
+      | _ -> false
+    in
+    let args =
+      List.mapi
+        (fun i a ->
+           if i < slots && is_type a then
+             { Ast.e = Ast.TypeArg (texpr a); loc = a.loc }
+           else expr a)
+        args
+    in
+    mk (Ast.Call (expr head, args))
+
   (* ── (array 4 rl/Vector2) ───────────────────────────────────────────
      A zeroed fixed array, told its count and its element type. The type
      spelling [4 rl/Vector2] is unchanged and still works everywhere a type is

test/programs/vec-new-type.flan

@@ -0,0 +1,25 @@
+;;;; vec-new and map-new take a type expression where they take a type, so a
+;;;; local can hold a Vec of slices, of arrays or of pointers with nothing
+;;;; else naming the element type. The last Vec names an allocator after its
+;;;; type, which is the one argument that may follow.
+
+(defn main [] i32
+  (let [a     (arena-new 4096)
+        words (vec-new [u8])
+        pairs (vec-new [2 i32])
+        ptrs  (vec-new (Ptr i32))
+        opts  (vec-new (Option i64) a)
+        m     (map-new string [u8])
+        x     (i32 7)]
+    (push words (bytes-view "ab"))
+    (push words (bytes-view "cde"))
+    (push pairs [3 4])
+    (push ptrs (addr x))
+    (push opts (Some (i64 9)))
+    (put m "k" (bytes-view "xyz"))
+    (println (length words) (length (at words 1))
+             (at (at pairs 0) 1) (deref (at ptrs 0))
+             (match (at opts 0) (Some v) v None -1)
+             (match (get m "k") (Some v) (length v) None -1))
+    (free words) (free pairs) (free ptrs) (free m))
+  0)

test/test_acceptance.ml

@@ -502,6 +502,12 @@ let () =
        rebinds every name at once, and interleaved writes would print 1. *)
     outputs "loop and recur" "programs/recur.flan"
       "10\n2\n21\n8\n10000000\n64\n012\n0\n4\n012\n6\n17\n";
+    (* A type expression where vec-new and map-new take a type: a slice, an
+       array, a pointer and an option, with nothing else naming the element. *)
+    outputs "vec-new takes a type expression" "programs/vec-new-type.flan"
+      "2 3 4 7 9 3\n";
+    outputs ~x86:true "vec-new takes a type expression, x86"
+      "programs/vec-new-type.flan" "2 3 4 7 9 3\n";
     (* into. The count of pulls is the assertion a unit test cannot make: one
        pass, one call per element per stage it reaches, and no intermediate
        collection anywhere. The two show lines either side of it are the same

test/test_flan.ml

@@ -6031,6 +6031,13 @@ let () =
   rejects_check "vec-new with no element type and nothing to take one from"
     ~needle:"nothing here says what (vec-new) is a Vec of"
     "(defn f [x $t] i32 (do x (let [v (vec-new)] (free v) 0)))";
+  (* A type expression in a type position, generic or not. *)
+  accepts "vec-new over a slice of a type variable"
+    "(defn f [x [$t]] i32 (let [v (vec-new [$t])] (push v x) \
+       (let [n (length v)] (free v) n)))";
+  rejects_check "map-new with a key type expression and no value type"
+    ~needle:"(map-new) names a key and no value"
+    "(defn f [] i32 (let [m (map-new [u8])] (free m) 0))";
   (* And a sigil on a name nothing binds is answered as the unbound variable
      it is, rather than as a missing element type — with the names that *are*
      bound, because inside a signature that introduces one the mistake is