feat: add File.copy and propagate effectful callback effects (WISH-7, WISH-14)

File.copy(source, dest) copies files via interpreter (std::fs::copy) and
C backend (fread/fwrite). Effectful callbacks passed to higher-order
functions like List.map/forEach now propagate their effects to the
enclosing function's inferred effect set.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/codegen/c_backend.rs

@@ -1149,6 +1149,7 @@ impl CBackend {
         self.writeln("    void (*delete_file)(void* env, LuxString path);");
         self.writeln("    LuxBool (*isDir)(void* env, LuxString path);");
         self.writeln("    void (*mkdir)(void* env, LuxString path);");
+        self.writeln("    void (*copy)(void* env, LuxString src, LuxString dst);");
         self.writeln("    LuxList* (*readDir)(void* env, LuxString path);");
         self.writeln("    void* env;");
         self.writeln("} LuxFileHandler;");
@@ -1336,6 +1337,20 @@ impl CBackend {
         self.writeln("    mkdir(path, 0755);");
         self.writeln("}");
         self.writeln("");
+        self.writeln("static void lux_file_copy(LuxString src, LuxString dst) {");
+        self.writeln("    FILE* fin = fopen(src, \"rb\");");
+        self.writeln("    if (!fin) return;");
+        self.writeln("    FILE* fout = fopen(dst, \"wb\");");
+        self.writeln("    if (!fout) { fclose(fin); return; }");
+        self.writeln("    char buf[4096];");
+        self.writeln("    size_t n;");
+        self.writeln("    while ((n = fread(buf, 1, sizeof(buf), fin)) > 0) {");
+        self.writeln("        fwrite(buf, 1, n, fout);");
+        self.writeln("    }");
+        self.writeln("    fclose(fin);");
+        self.writeln("    fclose(fout);");
+        self.writeln("}");
+        self.writeln("");
         self.writeln("#include <dirent.h>");
         self.writeln("// Forward declarations needed by lux_file_readDir");
         self.writeln("static LuxList* lux_list_new(int64_t capacity);");
@@ -1391,6 +1406,11 @@ impl CBackend {
         self.writeln("    lux_file_mkdir(path);");
         self.writeln("}");
         self.writeln("");
+        self.writeln("static void default_file_copy(void* env, LuxString src, LuxString dst) {");
+        self.writeln("    (void)env;");
+        self.writeln("    lux_file_copy(src, dst);");
+        self.writeln("}");
+        self.writeln("");
         self.writeln("static LuxList* default_file_readDir(void* env, LuxString path) {");
         self.writeln("    (void)env;");
         self.writeln("    return lux_file_readDir(path);");
@@ -1404,6 +1424,7 @@ impl CBackend {
         self.writeln("    .delete_file = default_file_delete,");
         self.writeln("    .isDir = default_file_isDir,");
         self.writeln("    .mkdir = default_file_mkdir,");
+        self.writeln("    .copy = default_file_copy,");
         self.writeln("    .readDir = default_file_readDir,");
         self.writeln("    .env = NULL");
         self.writeln("};");
@@ -3679,6 +3700,16 @@ impl CBackend {
                             }
                             return Ok("NULL".to_string());
                         }
+                        "copy" => {
+                            let src = self.emit_expr(&args[0])?;
+                            let dst = self.emit_expr(&args[1])?;
+                            if self.has_evidence {
+                                self.writeln(&format!("ev->file->copy(ev->file->env, {}, {});", src, dst));
+                            } else {
+                                self.writeln(&format!("lux_file_copy({}, {});", src, dst));
+                            }
+                            return Ok("NULL".to_string());
+                        }
                         "readDir" | "listDir" => {
                             let path = self.emit_expr(&args[0])?;
                             let temp = format!("_readdir_{}", self.fresh_name());
@@ -5175,7 +5206,7 @@ impl CBackend {
                 } else if effect.name == "File" {
                     match operation.name.as_str() {
                         "read" => Some("LuxString".to_string()),
-                        "write" | "append" | "delete" | "mkdir" => Some("void".to_string()),
+                        "write" | "append" | "delete" | "mkdir" | "copy" => Some("void".to_string()),
                         "exists" | "isDir" => Some("LuxBool".to_string()),
                         "readDir" | "listDir" => Some("LuxList*".to_string()),
                         _ => None,

src/interpreter.rs

@@ -3864,6 +3864,30 @@ impl Interpreter {
                 }
             }
 
+            ("File", "copy") => {
+                let source = match request.args.first() {
+                    Some(Value::String(s)) => s.clone(),
+                    _ => return Err(RuntimeError {
+                        message: "File.copy requires a string source path".to_string(),
+                        span: None,
+                    }),
+                };
+                let dest = match request.args.get(1) {
+                    Some(Value::String(s)) => s.clone(),
+                    _ => return Err(RuntimeError {
+                        message: "File.copy requires a string destination path".to_string(),
+                        span: None,
+                    }),
+                };
+                match std::fs::copy(&source, &dest) {
+                    Ok(_) => Ok(Value::Unit),
+                    Err(e) => Err(RuntimeError {
+                        message: format!("Failed to copy '{}' to '{}': {}", source, dest, e),
+                        span: None,
+                    }),
+                }
+            }
+
             // ===== Process Effect =====
             ("Process", "exec") => {
                 use std::process::Command;

src/main.rs

@@ -5629,4 +5629,55 @@ c")"#;
         "#;
         assert_eq!(eval(source).unwrap(), "Some(30)");
     }
+
+    #[test]
+    fn test_file_copy() {
+        use std::io::Write;
+        // Create a temp file, copy it, verify contents
+        let dir = std::env::temp_dir().join("lux_test_file_copy");
+        let _ = std::fs::create_dir_all(&dir);
+        let src = dir.join("src.txt");
+        let dst = dir.join("dst.txt");
+        std::fs::File::create(&src).unwrap().write_all(b"hello copy").unwrap();
+        let _ = std::fs::remove_file(&dst);
+
+        let source = format!(r#"
+            fn main(): Unit with {{File}} =
+                File.copy("{}", "{}")
+            let _ = run main() with {{}}
+            let result = "done"
+        "#, src.display(), dst.display());
+        let result = eval(&source);
+        assert!(result.is_ok(), "File.copy failed: {:?}", result);
+        let contents = std::fs::read_to_string(&dst).unwrap();
+        assert_eq!(contents, "hello copy");
+
+        // Cleanup
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
+    #[test]
+    fn test_effectful_callback_propagation() {
+        // WISH-7: effectful callbacks in List.forEach should propagate effects
+        // This should type-check successfully because Console effect is inferred
+        let source = r#"
+            fn printAll(items: List<String>): Unit =
+                List.forEach(items, fn(x: String): Unit => Console.print(x))
+            let result = "ok"
+        "#;
+        let result = eval(source);
+        assert!(result.is_ok(), "Effectful callback should type-check: {:?}", result);
+    }
+
+    #[test]
+    fn test_effectful_callback_in_map() {
+        // Effectful callback in List.map should propagate effects
+        let source = r#"
+            fn readAll(paths: List<String>): List<String> =
+                List.map(paths, fn(p: String): String => File.read(p))
+            let result = "ok"
+        "#;
+        let result = eval(source);
+        assert!(result.is_ok(), "Effectful callback in map should type-check: {:?}", result);
+    }
 }

src/typechecker.rs

@@ -1951,6 +1951,17 @@ impl TypeChecker {
         let func_type = self.infer_expr(func);
         let arg_types: Vec<Type> = args.iter().map(|a| self.infer_expr(a)).collect();
 
+        // Propagate effects from callback arguments to enclosing scope
+        for arg_type in &arg_types {
+            if let Type::Function { effects, .. } = arg_type {
+                for effect in &effects.effects {
+                    if self.inferring_effects {
+                        self.inferred_effects.insert(effect.clone());
+                    }
+                }
+            }
+        }
+
         // Check property constraints from where clauses
         if let Expr::Var(func_id) = func {
             if let Some(constraints) = self.property_constraints.get(&func_id.name).cloned() {
@@ -2061,6 +2072,18 @@ impl TypeChecker {
                 if let Some((_, field_type)) = fields.iter().find(|(n, _)| n == &operation.name) {
                     // It's a function call on a module field
                     let arg_types: Vec<Type> = args.iter().map(|a| self.infer_expr(a)).collect();
+
+                    // Propagate effects from callback arguments to enclosing scope
+                    for arg_type in &arg_types {
+                        if let Type::Function { effects, .. } = arg_type {
+                            for effect in &effects.effects {
+                                if self.inferring_effects {
+                                    self.inferred_effects.insert(effect.clone());
+                                }
+                            }
+                        }
+                    }
+
                     let result_type = Type::var();
                     let expected_fn = Type::function(arg_types, result_type.clone());
 
@@ -2120,6 +2143,17 @@ impl TypeChecker {
                 // Check argument types
                 let arg_types: Vec<Type> = args.iter().map(|a| self.infer_expr(a)).collect();
 
+                // Propagate effects from callback arguments to enclosing scope
+                for arg_type in &arg_types {
+                    if let Type::Function { effects, .. } = arg_type {
+                        for effect in &effects.effects {
+                            if self.inferring_effects {
+                                self.inferred_effects.insert(effect.clone());
+                            }
+                        }
+                    }
+                }
+
                 if arg_types.len() != op.params.len() {
                     self.errors.push(TypeError {
                         message: format!(

src/types.rs

@@ -956,6 +956,14 @@ impl TypeEnv {
                         params: vec![("path".to_string(), Type::String)],
                         return_type: Type::Unit,
                     },
+                    EffectOpDef {
+                        name: "copy".to_string(),
+                        params: vec![
+                            ("source".to_string(), Type::String),
+                            ("dest".to_string(), Type::String),
+                        ],
+                        return_type: Type::Unit,
+                    },
                 ],
             },
         );