hakorune/docs/reference/language/EBNF.md

Nyash Grammar (Stage‑2 EBNF)

Status: Fixed for Phase 15 Stage‑2. Parser implementations (Rust/Python/Nyash selfhost) should conform to this subset.

program := stmt* EOF

stmt := 'return' expr | 'local' IDENT '=' expr | 'if' expr block ('else' block)? | 'loop' '('? expr ')' ? block | expr ; expression statement

block := '{' stmt* '}'

expr := logic logic := compare (('&&' | '||') compare)* compare := sum (( '==' | '!=' | '<' | '>' | '<=' | '>=' ) sum)? sum := term (('+' | '-') term)* term := unary (('' | '/') unary) unary := '-' unary | factor

factor := INT | STRING | IDENT call_tail* | '(' expr ')' | 'new' IDENT '(' args? ')' | '[' args? ']' ; Array literal (Stage‑1 sugar, gated) | '{' map_entries? '}' ; Map literal (Stage‑2 sugar, gated) | match_expr ; Pattern matching (replaces legacy peek)

match_expr := 'match' expr '{' match_arm+ default_arm? '}' match_arm := pattern guard? '=>' (expr | block) ','? default_arm:= '_' '=>' (expr | block) ','?

pattern := '_' | STRING | INT | 'true' | 'false' | 'null' | IDENT '(' IDENT? ')' ; Type pattern e.g., StringBox(s) | '[' (IDENT (',' '..' IDENT)? )? ']' | '{' ( (STRING|IDENT) ':' IDENT (',' '..')? )? '}' | pattern '|' pattern ; OR pattern (same arm)

guard := 'if' expr

map_entries := (STRING | IDENT) ':' expr (',' (STRING | IDENT) ':' expr)* [',']

call_tail := '.' IDENT '(' args? ')' ; method | '(' args? ')' ; function call

args := expr (',' expr)*

Notes

• ASI: Newline is the primary statement separator. Do not insert a semicolon between a closed block and a following 'else'.
• Short-circuit: '&&' and '||' must not evaluate the RHS when not needed.
• Unary minus has higher precedence than '*' and '/'.
• IDENT names consist of [A-Za-z_][A-Za-z0-9_]*
• Array literal is enabled when syntax sugar is on (NYASH_SYNTAX_SUGAR_LEVEL=basic|full) or when NYASH_ENABLE_ARRAY_LITERAL=1 is set.
• Map literal is enabled when syntax sugar is on (NYASH_SYNTAX_SUGAR_LEVEL=basic|full) or when NYASH_ENABLE_MAP_LITERAL=1 is set.
• Identifier keys ({name: v}) are Stage‑3 and require either NYASH_SYNTAX_SUGAR_LEVEL=full or NYASH_ENABLE_MAP_IDENT_KEY=1.
• Pattern matching: match replaces legacy peek. MVP supports wildcard _, literals, simple type patterns, fixed/variadic array heads [hd, ..tl], simple map key extract { "k": v, .. }, OR patterns, and guards if.

Box Members (Phase‑15, env gate: NYASH_ENABLE_UNIFIED_MEMBERS; default ON)

This section adds a minimal grammar for Box members (a unified member model) without changing JSON v0/MIR. Parsing is controlled by env NYASH_ENABLE_UNIFIED_MEMBERS (default ON; set 0/false/off to disable).

box_decl       := 'box' IDENT '{' member* '}'

member         := stored
                | computed
                | once_decl
                | birth_once_decl
                | method_decl
                | block_as_role      ; nyash-mode (block-first) equivalent

stored         := IDENT ':' TYPE ( '=' expr )?
                  ; stored property (read/write). No handlers supported.

computed       := IDENT ':' TYPE ( '=>' expr | block ) handler_tail?
                  ; computed property (read‑only). Recomputes on each read.

once_decl      := 'once' IDENT ':' TYPE ( '=>' expr | block ) handler_tail?
                  ; lazy once. First read computes and caches; later reads return cached value.

birth_once_decl:= 'birth_once' IDENT ':' TYPE ( '=>' expr | block ) handler_tail?
                  ; eager once. Computed during construction (before user birth), in declaration order.

method_decl    := IDENT '(' params? ')' ( ':' TYPE )? block handler_tail?

; nyash-mode (block-first) variant — gated with NYASH_ENABLE_UNIFIED_MEMBERS=1
block_as_role  := block 'as' ( 'once' | 'birth_once' )? IDENT ':' TYPE

handler_tail   := ( catch_block )? ( cleanup_block )?
catch_block    := 'catch' ( '(' ( IDENT IDENT | IDENT )? ')' )? block
cleanup_block  := 'cleanup' block

Semantics (summary)

• stored: O(1) slot read; write via assignment. Initializer (if present) evaluates at construction once.
• computed: read‑only; each read evaluates the block; assignment is an error unless a setter is explicitly defined.
• once: first read evaluates the block and caches the value; subsequent reads return the cached value. On exception without a catch, the property becomes poisoned and rethrows on later reads (no retries).
• birth_once: evaluated before the user birth body, in declaration order; exceptions without a catch abort construction; cycles between birth_once members are an error.
• handlers: catch/cleanup are permitted for computed/once/birth_once/method blocks (Stage‑3), not for stored.

Lowering (no JSON v0 change)

• stored → slot
• computed → synthesize __get_name():T { try body; catch; finally }; reads of obj.name become obj.__get_name()
• once → add __name: Option<T> and emit __get_name() with first‑read initialization; on uncaught exception mark poisoned and rethrow on subsequent reads
• birth_once → add __name: T and insert initialization just before user birth in declaration order; handlers apply to each initializer
• method → existing method forms; optional postfix handlers lower to try/catch/finally

Stage‑3 (Gated) Additions

Enabled when NYASH_PARSER_STAGE3=1 for the Rust parser (and via --stage3/NYASH_NY_COMPILER_STAGE3=1 for the selfhost parser):

• try/catch/cleanup

  • try_stmt := 'try' block ('catch' '(' (IDENT IDENT | IDENT | ε) ')' block) ('cleanup' block)?
    • MVP policy: single catch per try。
    • (Type var) or (var) or () are accepted for the catch parameter。

• Block‑postfix catch/cleanup（Phase 15.5）

  • block_catch := '{' stmt* '}' ('catch' '(' (IDENT IDENT | IDENT | ε) ')' block)? ('cleanup' block)?
  • Applies to standalone block statements. Do not attach to if/else/loop structural blocks (wrap with a standalone block when needed).
  • Gate: NYASH_BLOCK_CATCH=1 (or NYASH_PARSER_STAGE3=1).

• throw

  • throw_stmt := 'throw' expr

• Method‑level postfix catch/cleanup（Phase 15.6, gated）

  • method_decl := 'method' IDENT '(' params? ')' block ('catch' '(' (IDENT IDENT | IDENT | ε) ')' block)? ('cleanup' block)?
  • Gate: NYASH_METHOD_CATCH=1（または NYASH_PARSER_STAGE3=1 と同梱）

• Member‑level postfix catch/cleanup（Phase 15.6, gated）

  • Applies to computed/once/birth_once in the unified member model: see “Box Members”.
  • Gate: NYASH_PARSER_STAGE3=1 (shared). Stored members do not accept handlers.

These constructs remain experimental; behaviour may degrade to no‑op in some backends until runtime support lands, as tracked in CURRENT_TASK.md.