(- x) negates a typed number, a type variable and a dyn, and a float's negation of zero is -0.0

TODO.org

@@ -143,8 +143,7 @@ CLOSED: [2026-09-25]
 =f64-inf=, =f64-nan=, =f32-inf= and =f32-nan= are names the checker supplies
 (=Check.special_float=), reached only after every local, global and function has
 missed, so a program's own binding of one wins. Negative infinity is
-=(- 0.0 f64-inf)=: the decision wrote =(- f64-inf)=, and there is no unary minus.
-Rules out Clojure's =##Inf= reader literal.
+=(- f64-inf)=. Rules out Clojure's =##Inf= reader literal.
 
 ** DONE A u64 constant above 2^63 cannot be written in decimal
 CLOSED: [2026-09-25]

lib/check.ml

@@ -2003,6 +2003,8 @@ let and_sentinel (x : Ast.expr) =
 let rec literal_arith (e : Ast.expr) : int64 option =
   match e.Ast.e with
   | Ast.Int n -> Some n
+  | Ast.Call ({ Ast.e = Ast.Var "-"; _ }, [ x ]) ->
+    Option.map Int64.neg (literal_arith x)
   | Ast.Call ({ Ast.e = Ast.Var op; _ }, x :: y :: rest) ->
     let step a b =
       match op with
@@ -6355,12 +6357,9 @@ and arity _ctx loc name n args =
    Two is the floor, and the two missing cases are refused rather than
    invented. Zero operands would have to mean an identity element, 0 for + and
    1 for *, and a sum with no terms in it is a typo far more often than it is
-   an intent. One operand would have to mean negation for [-] and reciprocal
-   for [/], and this language has no unary minus anywhere: the prelude writes
-   every negation as [(- 0 n)] or [(- 0.0 x)], and [(- x)] meaning something
-   else than the [-] two lines above it is a rule a reader has to carry rather
-   than see. Integer division makes the reciprocal worse still: [(/ 3)] would
-   be 0.
+   an intent. One operand is refused for every operator but [-], whose one
+   operand form is negation and is [named_call]'s. For [/] it would be the
+   reciprocal, and integer division makes that a trap: [(/ 3)] would be 0.
 
    A one-operand comparison would have to be [true] — there is no pair to
    disagree, and nothing for a lone value to be distinct from — and a test
@@ -6369,10 +6368,6 @@ and arity _ctx loc name n args =
 and fold_arity loc name args =
   match args with
   | _ :: _ :: _ -> ()
-  | [ _ ] when String.equal name "-" ->
-    fail loc
-      "- takes two arguments or more, given 1 — there is no unary minus; \
-       write (- 0 x) to negate"
   | [ _ ] when String.equal name "/" ->
     fail loc
       "/ takes two arguments or more, given 1 — there is no reciprocal; \
@@ -7029,6 +7024,31 @@ and named_call ?(qualified = false) ctx ~want loc name args =
   | _ when (not qualified) && shadows_builtin ctx loc name ->
     ordinary_call ctx ~want loc name args
   (* ── arithmetic and comparison ─────────────────────────────────── *)
+  (* (- x) negates, Clojure's rule. A literal operand is the negative literal,
+     so it takes its type from the site as any literal does. A float is
+     subtracted from -0.0, which is exact negation — 0.0 - 0.0 would answer
+     +0.0 — and an integer from 0, which wraps as (- 0 x) does. *)
+  | "-" when List.length args = 1 ->
+    let x = List.hd args in
+    (match x.Ast.e with
+     | Ast.Int n when n <> Int64.min_int ->
+       check ctx ?want { Ast.e = Ast.Int (Int64.neg n); loc }
+     | Ast.Float v -> check ctx ?want { Ast.e = Ast.Float (-.v); loc }
+     | _ ->
+       let v = check ctx ?want:(numeric_want want) x in
+       if v.Tast.ty = Types.Dyn then
+         expect ctx loc ~want (rt loc Types.Dyn "flan_dyn_neg" [ v; here loc ])
+       else begin
+         unconstrained ctx.env loc name ~needs:"numeric?" v.Tast.ty;
+         if not (Types.is_numeric v.Tast.ty || generic_ty v.Tast.ty) then
+           not_numeric name "numbers" v;
+         let zero =
+           match v.Tast.ty with
+           | Types.Float k -> mk loc v.Tast.ty (Tast.Float (-0.0, k))
+           | ty -> int_literal loc ~want:(Some ty) ~preds:ctx.env.tvpreds 0L
+         in
+         expect ctx loc ~want (mk loc v.Tast.ty (Tast.Prim (Tast.Sub, [ zero; v ])))
+       end)
   | "+" | "-" | "*" | "/" ->
     let p = match name with
       | "+" -> Tast.Add | "-" -> Tast.Sub | "*" -> Tast.Mul
@@ -10087,7 +10107,8 @@ let builtins : (string * string * string) list =
       numeric types meet at the wider one when that cannot lose — i32 and i64 \
       add at i64 — and i32 with u32 has no such type and is refused.");
     ("-", "- [numeric? ...] numeric?",
-     "Difference, folded left: (- a b c) is ((a - b) - c).");
+     "Difference, folded left: (- a b c) is ((a - b) - c). With one operand, \
+      its negation: (- x).");
     ("*", "* [numeric? ...] numeric?",
      "Product, folded left over two or more operands of one numeric type.");
     ("/", "/ [numeric? ...] numeric?",

lib/emit.ml

@@ -4389,6 +4389,7 @@ declare i64 @flan_dyn_sub(i64, i64, ptr, i64)
 declare i64 @flan_dyn_mul(i64, i64, ptr, i64)
 declare i64 @flan_dyn_div(i64, i64, ptr, i64)
 declare i64 @flan_dyn_rem(i64, i64, ptr, i64)
+declare i64 @flan_dyn_neg(i64, ptr, i64)
 declare i64 @flan_dyn_lt(i64, i64, ptr, i64)
 declare i64 @flan_dyn_le(i64, i64, ptr, i64)
 declare i64 @flan_dyn_gt(i64, i64, ptr, i64)

lib/prelude.ml

@@ -695,7 +695,7 @@ let source = {flan|
 ;; The floats are three questions and not two, which is why there is no
 ;; f32-min here to sit beside f32-max.
 ;;
-;; A float's least value is just the negation of its greatest — (- 0.0 f32-max)
+;; A float's least value is just the negation of its greatest — (- f32-max)
 ;; — so a constant for it would say nothing the language cannot. What a caller
 ;; actually reaches for under the name "min" is the smallest positive one, and
 ;; that is a different number entirely. Naming it f32-min would make the two

runtime/flan_dyn.c

@@ -1759,6 +1759,14 @@ flan_dyn flan_dyn_sub(flan_dyn a, flan_dyn b, const uint8_t *loc,
                       int64_t loclen) {
   return arith(loc, loclen, "-", a, b);
 }
+/* (- x): an int wraps, as (- 0 x) does, and a float flips its sign, so the
+ * negation of 0.0 is -0.0 and not the 0.0 a subtraction from zero gives. */
+flan_dyn flan_dyn_neg(flan_dyn a, const uint8_t *loc, int64_t loclen) {
+  if (!is_num(a)) trap1(loc, loclen, TYPE_TRAP, "-", "it takes a number", a);
+  if (flan_dyn_tag(a) == FLAN_DYN_TAG_INT)
+    return flan_dyn_from_i64((int64_t)(0 - (uint64_t)dyn_int_value(a)));
+  return flan_dyn_from_f64(-dyn_num_value(a));
+}
 flan_dyn flan_dyn_mul(flan_dyn a, flan_dyn b, const uint8_t *loc,
                       int64_t loclen) {
   return arith(loc, loclen, "*", a, b);

runtime/flan_dyn.h

@@ -151,6 +151,7 @@ flan_dyn flan_dyn_sub(flan_dyn a, flan_dyn b, const uint8_t *loc, int64_t loclen
 flan_dyn flan_dyn_mul(flan_dyn a, flan_dyn b, const uint8_t *loc, int64_t loclen);
 flan_dyn flan_dyn_div(flan_dyn a, flan_dyn b, const uint8_t *loc, int64_t loclen);
 flan_dyn flan_dyn_rem(flan_dyn a, flan_dyn b, const uint8_t *loc, int64_t loclen);
+flan_dyn flan_dyn_neg(flan_dyn a, const uint8_t *loc, int64_t loclen);
 
 /* Answer a bool dyn. Numbers compare as numbers and text compares bytewise;
  * a mixture of the two, or anything else, traps. */

test/programs/limits.flan

@@ -119,17 +119,17 @@
   ;; the absence is recorded rather than merely unmentioned: a float's least
   ;; value is the negation of its greatest, and there is nothing to derive.
   (say "f32's least value negates its greatest"
-       (< (- (f32 0.0) f32-max) (- (f32 0.0) f32-min-positive)))
+       (< (- f32-max) (- f32-min-positive)))
   (say "f64's least value negates its greatest"
-       (< (- 0.0 f64-max) (- 0.0 f64-min-positive)))
+       (< (- f64-max) (- f64-min-positive)))
 
   ;; The infinities and NaNs, which no literal writes. Each infinity is the
   ;; overflow of its type's greatest value, negated it is below the least
   ;; finite one, and a NaN is the one value not equal to itself.
   (say "f64-inf" (= f64-inf (* f64-max 2.0)))
   (say "f32-inf" (= f32-inf (* f32-max (f32 2.0))))
-  (say "f64-inf negated" (< (- 0.0 f64-inf) (- 0.0 f64-max)))
-  (say "f32-inf negated" (< (- (f32 0.0) f32-inf) (- (f32 0.0) f32-max)))
+  (say "f64-inf negated" (< (- f64-inf) (- f64-max)))
+  (say "f32-inf negated" (< (- f32-inf) (- f32-max)))
   (say "f64-nan" (not (= f64-nan f64-nan)))
   (say "f32-nan" (not (= f32-nan f32-nan)))
   ;; != is the one unordered comparison: a NaN is unequal to everything,

test/programs/negate.flan

@@ -0,0 +1,25 @@
+;;;; (- x) negates: an integer wraps, a float flips its sign — the negation of
+;;;; 0.0 is -0.0, which 1/x tells apart — and a dyn does either by its tag.
+(defn negi [x i32] i32 (- x))
+(defn negf [x f64] f64 (- x))
+(defn negf32 [x f32] f32 (- x))
+(defn negu [x u8] u8 (- x))
+(defn negd [x dyn] dyn (- x))
+(defn negg [x $t] $t {:where (numeric? $t)} (- x))
+(defn main [] i32
+  (println (negi 3))
+  (println (negi -7))
+  (println (negf 2.5))
+  (println (/ 1.0 (negf 0.0)))
+  (println (negf32 (f32 1.5)))
+  (println (negu (u8 1)))
+  (println (negd 4))
+  (println (negd 2.5))
+  (println (/ 1.0 (negd 0.0)))
+  (println (negg (i64 9000000000)))
+  (println (negg 0.5))
+  (let [a (- 5) b (i64 (- 3))]
+    (println (+ a (i32 b))))
+  (println (- f64-inf))
+  (println (< (- f64-inf) (- f64-max)))
+  0)

test/test_acceptance.ml

@@ -558,6 +558,13 @@ let () =
       "programs/array-first-element.flan" first_out;
     outputs ~x86:true "an array literal's first element types the rest, x86"
       "programs/array-first-element.flan" first_out;
+    (* (- x) negates, on every numeric type, a type variable and a dyn. *)
+    let neg_out =
+      "-3\n7\n-2.5\n-inf\n-1.5\n255\n-4\n-2.5\n-inf\n-9000000000\n\
+       -0.5\n-8\n-inf\ntrue\n" in
+    outputs "unary minus" "programs/negate.flan" neg_out;
+    outputs ~opt:"-O0" "unary minus, -O0" "programs/negate.flan" neg_out;
+    outputs ~x86:true "unary minus, x86" "programs/negate.flan" neg_out;
     (* A literal arm takes the other arm's type. *)
     let arm_out =
       "4000000\n9000000000\n5000000000\n7\n9000000000\n3\n9000000000\n2.5\n" in

test/test_flan.ml

@@ -1300,14 +1300,18 @@ let () =
   infers "min at four"       "(min 4 1 3 2)" "i32";
   infers "max at four"       "(max 4 1 3 2)" "i32";
   (* And the two counts below the floor. Zero would have to mean an identity
-     element and one a unary operator this language does not have; both are a
-     typo far more often than an intent, so both are refused by name. *)
+     element, and one is refused for every operator but -, whose one-operand
+     form negates. *)
   rejects_check "a sum with no terms"
     "(defn f [] i32 (+))" ~needle:"+ takes two arguments or more, given 0";
   rejects_check "a product with no factors"
     "(defn f [] i32 (*))" ~needle:"* takes two arguments or more, given 0";
-  rejects_check "there is no unary minus"
-    "(defn f [] i32 (- 1))" ~needle:"there is no unary minus";
+  infers "a negated literal" "(- 1)" "i32";
+  infers "a negated literal takes its type from the site" "(i64 (- 1))" "i64";
+  rejects_check "unary minus over a string names the operand"
+    "(defn f [s string] () (println (- s)))" ~needle:"- takes numbers";
+  rejects_check "unary minus at an unsigned literal is out of range"
+    "(defn f [] u8 (- 1))" ~needle:"does not fit in u8";
   rejects_check "there is no reciprocal"
     "(defn f [] f64 (/ 2.0))" ~needle:"there is no reciprocal";
   rejects_check "one operand is not a bitwise and"