feat: add effects to JS backend and fix code generation bugs

Phase 2 of JS backend: implement effect handlers in runtime

Effects added:
- Console: print, readLine, readInt
- Random: int, bool, float
- Time: now, sleep
- Http: get, post, postJson (async with fetch)

Bug fixes:
- Fix if-else with blocks executing both branches (use if-else
  statement instead of ternary for branches with statements)
- Fix main function being called twice when top-level let binding
  already invokes it
- Fix List module operations incorrectly treated as effect operations

New tests:
- test_js_random_int
- test_js_random_bool
- test_js_random_float
- test_js_time_now

All 19 JS backend tests pass.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/codegen/js_backend.rs

@@ -130,16 +130,27 @@ impl JsBackend {
             }
         }
 
+        // Check if any top-level let calls main (to avoid double invocation)
+        let has_main_call = program.declarations.iter().any(|decl| {
+            if let Declaration::Let(l) = decl {
+                self.expr_calls_main(&l.value)
+            } else {
+                false
+            }
+        });
+
         // Emit top-level let bindings and expressions
+        if program.declarations.iter().any(|d| matches!(d, Declaration::Let(_))) {
             self.writeln("// Top-level bindings");
             for decl in &program.declarations {
                 if let Declaration::Let(l) = decl {
                     self.emit_top_level_let(l)?;
                 }
             }
+        }
 
-        // Emit main function call if it exists
+        // Emit main function call if it exists and not already called
-        if self.functions.contains("main") {
+        if self.functions.contains("main") && !has_main_call {
             self.writeln("");
             self.writeln("// Entry point");
             if self.effectful_functions.contains("main") {
@@ -176,18 +187,107 @@ impl JsBackend {
         // Default handlers for effects
         self.writeln("defaultHandlers: {");
         self.indent += 1;
+
+        // Console effect
         self.writeln("Console: {");
         self.indent += 1;
         self.writeln("print: (msg) => console.log(msg),");
-        self.writeln("readLine: () => { throw new Error('readLine not supported in browser'); },");
+        self.writeln("readLine: () => {");
-        self.writeln("readInt: () => { throw new Error('readInt not supported in browser'); }");
-        self.indent -= 1;
-        self.writeln("},");
-        self.writeln("Random: {");
         self.indent += 1;
-        self.writeln("int: (min, max) => Math.floor(Math.random() * (max - min + 1)) + min");
+        self.writeln("if (typeof require !== 'undefined') {");
+        self.indent += 1;
+        self.writeln("const readline = require('readline');");
+        self.writeln("const rl = readline.createInterface({ input: process.stdin, output: process.stdout });");
+        self.writeln("return new Promise(resolve => rl.question('', answer => { rl.close(); resolve(answer); }));");
         self.indent -= 1;
         self.writeln("}");
+        self.writeln("return prompt('') || '';");
+        self.indent -= 1;
+        self.writeln("},");
+        self.writeln("readInt: () => parseInt(Lux.defaultHandlers.Console.readLine(), 10)");
+        self.indent -= 1;
+        self.writeln("},");
+
+        // Random effect
+        self.writeln("Random: {");
+        self.indent += 1;
+        self.writeln("int: (min, max) => Math.floor(Math.random() * (max - min + 1)) + min,");
+        self.writeln("bool: () => Math.random() < 0.5,");
+        self.writeln("float: () => Math.random()");
+        self.indent -= 1;
+        self.writeln("},");
+
+        // Time effect
+        self.writeln("Time: {");
+        self.indent += 1;
+        self.writeln("now: () => Date.now(),");
+        self.writeln("sleep: (ms) => new Promise(resolve => setTimeout(resolve, ms))");
+        self.indent -= 1;
+        self.writeln("},");
+
+        // Http effect (browser/Node compatible)
+        self.writeln("Http: {");
+        self.indent += 1;
+        self.writeln("get: async (url) => {");
+        self.indent += 1;
+        self.writeln("try {");
+        self.indent += 1;
+        self.writeln("const response = await fetch(url);");
+        self.writeln("const body = await response.text();");
+        self.writeln("const headers = [];");
+        self.writeln("response.headers.forEach((v, k) => headers.push([k, v]));");
+        self.writeln("return Lux.Ok({ status: response.status, body, headers });");
+        self.indent -= 1;
+        self.writeln("} catch (e) {");
+        self.indent += 1;
+        self.writeln("return Lux.Err(e.message);");
+        self.indent -= 1;
+        self.writeln("}");
+        self.indent -= 1;
+        self.writeln("},");
+        self.writeln("post: async (url, body) => {");
+        self.indent += 1;
+        self.writeln("try {");
+        self.indent += 1;
+        self.writeln("const response = await fetch(url, { method: 'POST', body });");
+        self.writeln("const respBody = await response.text();");
+        self.writeln("const headers = [];");
+        self.writeln("response.headers.forEach((v, k) => headers.push([k, v]));");
+        self.writeln("return Lux.Ok({ status: response.status, body: respBody, headers });");
+        self.indent -= 1;
+        self.writeln("} catch (e) {");
+        self.indent += 1;
+        self.writeln("return Lux.Err(e.message);");
+        self.indent -= 1;
+        self.writeln("}");
+        self.indent -= 1;
+        self.writeln("},");
+        self.writeln("postJson: async (url, json) => {");
+        self.indent += 1;
+        self.writeln("try {");
+        self.indent += 1;
+        self.writeln("const response = await fetch(url, {");
+        self.indent += 1;
+        self.writeln("method: 'POST',");
+        self.writeln("headers: { 'Content-Type': 'application/json' },");
+        self.writeln("body: JSON.stringify(json)");
+        self.indent -= 1;
+        self.writeln("});");
+        self.writeln("const body = await response.text();");
+        self.writeln("const headers = [];");
+        self.writeln("response.headers.forEach((v, k) => headers.push([k, v]));");
+        self.writeln("return Lux.Ok({ status: response.status, body, headers });");
+        self.indent -= 1;
+        self.writeln("} catch (e) {");
+        self.indent += 1;
+        self.writeln("return Lux.Err(e.message);");
+        self.indent -= 1;
+        self.writeln("}");
+        self.indent -= 1;
+        self.writeln("}");
+        self.indent -= 1;
+        self.writeln("}");
+
         self.indent -= 1;
         self.writeln("}");
 
@@ -410,11 +510,37 @@ impl JsBackend {
                 else_branch,
                 ..
             } => {
+                // Check if branches contain statements that need if-else instead of ternary
+                let needs_block = self.expr_has_statements(then_branch)
+                    || self.expr_has_statements(else_branch);
+
+                if needs_block {
+                    // Use if-else statement for branches with statements
+                    let result_var = format!("_if_result_{}", self.fresh_name());
+                    let cond = self.emit_expr(condition)?;
+
+                    self.writeln(&format!("let {};", result_var));
+                    self.writeln(&format!("if ({}) {{", cond));
+                    self.indent += 1;
+                    let then_val = self.emit_expr(then_branch)?;
+                    self.writeln(&format!("{} = {};", result_var, then_val));
+                    self.indent -= 1;
+                    self.writeln("} else {");
+                    self.indent += 1;
+                    let else_val = self.emit_expr(else_branch)?;
+                    self.writeln(&format!("{} = {};", result_var, else_val));
+                    self.indent -= 1;
+                    self.writeln("}");
+
+                    Ok(result_var)
+                } else {
+                    // Simple ternary for pure expressions
                     let cond = self.emit_expr(condition)?;
                     let then_val = self.emit_expr(then_branch)?;
                     let else_val = self.emit_expr(else_branch)?;
                     Ok(format!("({} ? {} : {})", cond, then_val, else_val))
                 }
+            }
 
             Expr::Let {
                 name, value, body, ..
@@ -983,6 +1109,48 @@ impl JsBackend {
         }
     }
 
+    /// Check if an expression contains statements that emit code before the result
+    fn expr_has_statements(&self, expr: &Expr) -> bool {
+        match expr {
+            Expr::Block { statements, .. } => !statements.is_empty(),
+            Expr::Let { .. } => true,
+            Expr::Match { .. } => true,
+            Expr::If { then_branch, else_branch, .. } => {
+                self.expr_has_statements(then_branch) || self.expr_has_statements(else_branch)
+            }
+            _ => false,
+        }
+    }
+
+    /// Check if an expression calls the main function
+    fn expr_calls_main(&self, expr: &Expr) -> bool {
+        match expr {
+            Expr::Call { func, .. } => {
+                if let Expr::Var(ident) = func.as_ref() {
+                    if ident.name == "main" {
+                        return true;
+                    }
+                }
+                false
+            }
+            Expr::Run { expr, .. } => self.expr_calls_main(expr),
+            Expr::Let { value, body, .. } => {
+                self.expr_calls_main(value) || self.expr_calls_main(body)
+            }
+            Expr::Block { statements, result, .. } => {
+                for stmt in statements {
+                    if let Statement::Let { value, .. } = stmt {
+                        if self.expr_calls_main(value) {
+                            return true;
+                        }
+                    }
+                }
+                self.expr_calls_main(result)
+            }
+            _ => false,
+        }
+    }
+
     /// Mangle a Lux name to a valid JavaScript name
     fn mangle_name(&self, name: &str) -> String {
         format!("{}_lux", name)
@@ -1265,4 +1433,60 @@ mod tests {
         let output = compile_and_run(source).expect("Should compile and run");
         assert_eq!(output, "Hello, World!");
     }
+
+    #[test]
+    fn test_js_random_int() {
+        let source = r#"
+            fn main(): Unit with {Console, Random} = {
+                let n = Random.int(1, 10)
+                // Just verify it runs without error and produces a number
+                Console.print("Random: " + toString(n))
+            }
+        "#;
+
+        let output = compile_and_run(source).expect("Should compile and run");
+        assert!(output.starts_with("Random: "));
+    }
+
+    #[test]
+    fn test_js_random_bool() {
+        let source = r#"
+            fn main(): Unit with {Console, Random} = {
+                let b = Random.bool()
+                Console.print("Bool: " + toString(b))
+            }
+        "#;
+
+        let output = compile_and_run(source).expect("Should compile and run");
+        assert!(output == "Bool: true" || output == "Bool: false");
+    }
+
+    #[test]
+    fn test_js_random_float() {
+        let source = r#"
+            fn main(): Unit with {Console, Random} = {
+                let f = Random.float()
+                // Float should be between 0 and 1
+                Console.print("Float generated")
+            }
+        "#;
+
+        let output = compile_and_run(source).expect("Should compile and run");
+        assert_eq!(output, "Float generated");
+    }
+
+    #[test]
+    fn test_js_time_now() {
+        let source = r#"
+            fn main(): Unit with {Console, Time} = {
+                let t = Time.now()
+                // Should be a positive timestamp
+                if t > 0 then Console.print("Time works")
+                else Console.print("Time failed")
+            }
+        "#;
+
+        let output = compile_and_run(source).expect("Should compile and run");
+        assert_eq!(output, "Time works");
+    }
 }