fixed #288

Author: arnog

CHANGELOG.md

@@ -11,6 +11,14 @@
   `Number.MAX_SAFE_INTEGER`) are now automatically promoted to `BigInt` to
   maintain exact results.
 
+### Symbols
+
+- **[\#288](https://github.com/cortex-js/compute-engine/issues/288) Allow
+  reassigning a symbol from operator to value**: `ce.assign()` no longer throws
+  when assigning a plain value to a symbol that was previously declared as a
+  function. Existing expressions using the symbol as a function head will
+  produce a type error at evaluation time if the new value is not callable.
+
 ### Evaluation
 
 - **Fixed scope leaks**: Ensured that evaluation contexts are correctly popped

src/compute-engine/engine-declarations.ts

@@ -295,18 +295,19 @@ export function assignFn(
 
   if (isOperatorDef(def)) {
     const value = assignValueAsValue(ce, arg2);
-    if (value) throw Error(`Cannot change the operator "${id}" to a value`);
+    if (value !== undefined) {
+      // Allow converting an operator to a value.
+      // Existing expressions using this symbol as a function head (e.g.
+      // ["g", 2]) will produce a type error at evaluation time if the
+      // new value is not callable — which is the correct semantic.
+      updateDef(ce, id, def, { value });
+      ce._setSymbolValue(id, value);
+      return ce;
+    }
 
     // Update the operator definition.
-    // Note: this is a potentially dangerous operation, since the
-    // operator may be used in other expressions that are not
-    // re-canonicalized. However, it might be desirable to override the
-    // evaluation of an operator in a specific context, even a system
-    // operator.
-    // However, it is preferable to use `ce.declare()` to create a new
-    // operator.
     const fnDef = assignValueAsOperatorDef(ce, arg2);
-    if (!fnDef) throw Error(`Cannot change the operator "${id}" to a value`);
+    if (!fnDef) throw Error(`Invalid definition for symbol "${id}"`);
     updateDef(ce, id, def, fnDef);
     return ce;
   }

test/compute-engine/functions.test.ts

@@ -223,9 +223,12 @@ describe('Changing type from function to non-function', () => {
     engine.pushScope();
     engine.declare('f20', 'any');
     engine.assign('f20', ['Function', ['Multiply', 'x', 2], 'x']);
-    expect(() => engine.assign('f20', 42)).toThrow();
+    expect(engine.box(['f20', 3]).evaluate().re).toEqual(6);
+
+    // Reassigning from operator to value is allowed (#288)
+    engine.assign('f20', 42);
+    expect(engine.box('f20').evaluate().re).toEqual(42);
 
-    // expect(engine.box('f20').re).toEqual(42);
     engine.popScope();
   });
 });