gat-min Language Specification (Bootstrap Subset)

gat-min is the minimal frozen subset of the gat programming language used to implement the self-hosted compiler gatc-v1.

---

1. Features Included in gat-min

1.1 Types

• Primitives: i64, bool, void, string
• Value Structs: struct Name { f1: T; f2: U; } (stack allocated, passed/copied by value)
• Reference Classes: class Name { f1: T; ... deinit { ... } } (heap allocated via new, non-atomic ARC managed, optional RAII deinit)
• Unsafe Raw Pointers: raw T (e.g. raw i8, raw i64, raw Point), supporting index access ptr[idx] and address-of raw expr

1.2 Declarations & Functions

• Top-level struct, class, fn
• Functions with typed parameters and explicit return types (fn name(p1: T, p2: U) -> Ret)
• Calling convention: borrowed by default for reference class parameters

1.3 Statements & Control Flow

• Variable declarations: let name = expr; / let name: Type = expr;
• Variable & field assignments: x = expr;, obj.field = expr;, buf[idx] = expr;
• Conditional branching: if (cond) { ... } else { ... }
• Iteration: while (cond) { ... }
• Function returns: return; / return expr;
• Expressions as statements: function calls, print(...)

1.4 Operators & Expressions

• Arithmetic: +, -, *, /, %, unary -
• Relational & Equality: ==, !=, <, <=, >, >=
• Logical: &&, ||, !
• Member Access: obj.field
• Index Access: buf[index] (loads byte if raw i8/string, 8 bytes if raw i64/general pointer)
• Instantiation: new ClassName { field: val, ... } and StructName { field: val, ... }
• Literals: decimal integers (123), hex integers (0x7F), character literals ('a', '\n'), strings ("..."), booleans (true, false), nil

1.5 Builtin Runtime Intrinsics

• print(args...): Output strings, numbers, booleans
• read_file(path: string) -> string: Read whole file from disk
• write_file(path: string, data: string, len: i64) -> i64: Write bytes to disk
• alloc_mem(bytes: i64) -> raw i8: Allocate raw heap buffer
• free_mem(ptr: raw i8): Free raw heap buffer
• str_len(s: string) -> i64: Get string length
• str_eq(a: string, b: string) -> bool: Compare two strings
• str_char(s: string, idx: i64) -> i64: Read ASCII byte at index
• str_sub(s: string, start: i64, len: i64) -> string: Substring extraction
• str_concat(a: string, b: string) -> string: Concatenate two strings
• str_from_int(n: i64) -> string: Convert integer to string
• get_cmd_arg(idx: i64) -> string: Retrieve command line argument

---

2. Features Implemented Post-Bootstrap (in Full gat)

• Generics: Uniform 64-bit word-sized generic type erasure (class Pair<T, U>, functions)
• First-Class Functions: Function pointer values and function types fn(T1, T2) -> TRet
• Pattern Matching: Exhaustive match on tagged enums
• Cycle Collection: weak T non-owning references and weak_upgrade
• Concurrency: Native OS threading (std/thread.gat) and synchronization (std/sync.gat Mutex) with compile-time thread-boundary reference isolation

3. Deliberately Excluded Features (Current Scope Boundary)

• Full heap-captured closures (anonymous closures capturing outer scope)
• Shared mutable class reference graphs across threads (thread-local heaps enforced)
• Trait / Interface dynamic dispatch tables