cypher-codegen: account for fallback argument types in coalesce inference

.changeset/coalesce-fallback-inference.md

@@ -0,0 +1,5 @@
+---
+"@evryg/effect-cypher-codegen": patch
+---
+
+coalesce now accounts for fallback argument types when the leading argument is nullable. Previously the result type was the leading argument's type stripped of nullable, ignoring the fallbacks. When a nullable Double property is coalesced with an integer literal (`coalesce(n.score, 0)`), the runtime yields that integer literal as a database Integer when the property is null — which the Double decoder rejects. The result now widens to Long (the integer-tolerant numeric superset, which also passes floats through), so the emitted column decodes correctly. Same-type and non-nullable leading arguments are unaffected.

packages/cypher-codegen/src/frontend/InferType.node.unit.test.ts

@@ -41,7 +41,23 @@ const schema = new GraphSchema({
       mandatory: false
     }),
     new VertexProperty({ labels: ["Method"], propertyName: "returnType", propertyTypes: ["String"], mandatory: false }),
-    new VertexProperty({ labels: ["Method"], propertyName: "ccn", propertyTypes: ["Long"], mandatory: false })
+    new VertexProperty({ labels: ["Method"], propertyName: "ccn", propertyTypes: ["Long"], mandatory: false }),
+    // Sample is a synthetic fixture for type-inference tests: its properties are named by their type
+    // and nullability role, not by any domain concept.
+    new VertexProperty({
+      labels: ["Sample"],
+      propertyName: "nullableDouble",
+      propertyTypes: ["Double"],
+      mandatory: false
+    }),
+    new VertexProperty({ labels: ["Sample"], propertyName: "nullableLong", propertyTypes: ["Long"], mandatory: false }),
+    new VertexProperty({ labels: ["Sample"], propertyName: "requiredLong", propertyTypes: ["Long"], mandatory: true }),
+    new VertexProperty({
+      labels: ["Sample"],
+      propertyName: "requiredString",
+      propertyTypes: ["String"],
+      mandatory: true
+    })
   ],
   edgeProperties: [
     new EdgeProperty({ edgeType: "CALLS", propertyName: "confidence", propertyTypes: ["String"], mandatory: true }),
@@ -180,6 +196,33 @@ describe("inferExpressionType — coalesce", () => {
     const result = inferExpressionType(parseExpression("coalesce(f.lineCount, 0)"), env, schema)
     expect(result).toEqual(new ScalarType({ scalarType: "Long" }))
   })
+
+  it("nullable Double coalesced with an integer literal infers Long", () => {
+    // coalesce(<nullable Double>, 0): when the property is null the runtime returns the integer
+    // literal as a database Integer object, which only the integer-tolerant decoder accepts. The
+    // fallback's integer type must widen the result to Long, not stay Double.
+    const env = envWith({ s: { type: new VertexType({ label: "Sample" }), nullable: false } })
+    const result = inferExpressionType(parseExpression("coalesce(s.nullableDouble, 0)"), env, schema)
+    expect(result).toEqual(new ScalarType({ scalarType: "Long" }))
+  })
+
+  it("nullable Long coalesced with an integer literal stays Long", () => {
+    const env = envWith({ s: { type: new VertexType({ label: "Sample" }), nullable: false } })
+    const result = inferExpressionType(parseExpression("coalesce(s.nullableLong, 0)"), env, schema)
+    expect(result).toEqual(new ScalarType({ scalarType: "Long" }))
+  })
+
+  it("non-null Long coalesced with an integer literal stays Long", () => {
+    const env = envWith({ s: { type: new VertexType({ label: "Sample" }), nullable: false } })
+    const result = inferExpressionType(parseExpression("coalesce(s.requiredLong, 0)"), env, schema)
+    expect(result).toEqual(new ScalarType({ scalarType: "Long" }))
+  })
+
+  it("non-null String coalesced with a string literal stays String", () => {
+    const env = envWith({ s: { type: new VertexType({ label: "Sample" }), nullable: false } })
+    const result = inferExpressionType(parseExpression("coalesce(s.requiredString, \"x\")"), env, schema)
+    expect(result).toEqual(new ScalarType({ scalarType: "String" }))
+  })
 })
 
 describe("inferExpressionType — size", () => {

packages/cypher-codegen/src/frontend/InferType.ts

@@ -589,6 +589,21 @@ function extractIsNotNullVar(expr: ExpressionContext): string | undefined {
   return symbol.getText()
 }
 
+/**
+ * Unify two coalesce candidate types (already stripped of nullable). Equal scalars collapse to that
+ * scalar; two numeric scalars (Long/Double in any mix) collapse to Long — the integer-tolerant
+ * decoder is the numeric superset, accepting both database integers and floats. Anything else keeps
+ * the leading argument's type (no regression for heterogeneous or non-scalar candidates).
+ */
+function unifyCoalesce(a: CypherType, b: CypherType): CypherType {
+  if (a._tag === "ScalarType" && b._tag === "ScalarType") {
+    if (a.scalarType === b.scalarType) return a
+    const isNumeric = (s: string) => s === "Long" || s === "Double"
+    if (isNumeric(a.scalarType) && isNumeric(b.scalarType)) return new ScalarType({ scalarType: "Long" })
+  }
+  return a
+}
+
 function inferFunctionType(
   func: FunctionInvocationContext,
   env: TypeEnv,
@@ -608,13 +623,21 @@ function inferFunctionType(
     throw new CypherTypeError("collect() requires an argument")
   }
 
-  // coalesce(x, ...) → type of first arg, stripped of nullable (coalesce provides fallback)
+  // coalesce(x, ...) → coalesce returns the first non-null argument, so the result type unifies the
+  // candidates left→right, each stripped of nullable. Walk arguments until the first non-nullable one
+  // (later arguments are unreachable). An integer-literal fallback for a nullable Double widens the
+  // result to Long: when the property is null the runtime yields that integer literal, and only the
+  // integer-tolerant decoder accepts it (it also passes floats through unchanged).
   if (funcName === "coalesce") {
-    if (args.length > 0) {
-      const argType = inferExpressionType(args[0], env, schema)
-      return argType._tag === "NullableType" ? argType.inner : argType
+    if (args.length === 0) throw new CypherTypeError("coalesce() requires arguments")
+    let result: CypherType | undefined
+    for (const arg of args) {
+      const argType = inferExpressionType(arg, env, schema)
+      const stripped = argType._tag === "NullableType" ? argType.inner : argType
+      result = result === undefined ? stripped : unifyCoalesce(result, stripped)
+      if (argType._tag !== "NullableType") break
     }
-    throw new CypherTypeError("coalesce() requires arguments")
+    return result!
   }
 
   // properties(x) → Map with unknown fields