159 lines
5.0 KiB
OCaml
159 lines
5.0 KiB
OCaml
(*===---------------------------------------------------------------------===
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* Parser
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*===---------------------------------------------------------------------===*)
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(* binop_precedence - This holds the precedence for each binary operator that is
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* defined *)
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let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
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(* precedence - Get the precedence of the pending binary operator token. *)
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let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1
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(* primary
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* ::= identifier
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* ::= numberexpr
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* ::= parenexpr
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* ::= ifexpr
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* ::= forexpr *)
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let rec parse_primary = parser
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(* numberexpr ::= number *)
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| [< 'Token.Number n >] -> Ast.Number n
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(* parenexpr ::= '(' expression ')' *)
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| [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e
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(* identifierexpr
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* ::= identifier
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* ::= identifier '(' argumentexpr ')' *)
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| [< 'Token.Ident id; stream >] ->
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let rec parse_args accumulator = parser
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| [< e=parse_expr; stream >] ->
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begin parser
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| [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e
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| [< >] -> e :: accumulator
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end stream
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| [< >] -> accumulator
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in
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let rec parse_ident id = parser
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(* Call. *)
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| [< 'Token.Kwd '(';
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args=parse_args [];
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'Token.Kwd ')' ?? "expected ')'">] ->
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Ast.Call (id, Array.of_list (List.rev args))
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(* Simple variable ref. *)
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| [< >] -> Ast.Variable id
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in
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parse_ident id stream
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(* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
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| [< 'Token.If; c=parse_expr;
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'Token.Then ?? "expected 'then'"; t=parse_expr;
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'Token.Else ?? "expected 'else'"; e=parse_expr >] ->
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Ast.If (c, t, e)
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(* forexpr
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::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *)
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| [< 'Token.For;
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'Token.Ident id ?? "expected identifier after for";
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'Token.Kwd '=' ?? "expected '=' after for";
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stream >] ->
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begin parser
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| [<
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start=parse_expr;
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'Token.Kwd ',' ?? "expected ',' after for";
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end_=parse_expr;
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stream >] ->
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let step =
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begin parser
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| [< 'Token.Kwd ','; step=parse_expr >] -> Some step
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| [< >] -> None
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end stream
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in
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begin parser
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| [< 'Token.In; body=parse_expr >] ->
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Ast.For (id, start, end_, step, body)
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| [< >] ->
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raise (Stream.Error "expected 'in' after for")
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end stream
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| [< >] ->
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raise (Stream.Error "expected '=' after for")
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end stream
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| [< >] -> raise (Stream.Error "unknown token when expecting an expression.")
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(* binoprhs
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* ::= ('+' primary)* *)
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and parse_bin_rhs expr_prec lhs stream =
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match Stream.peek stream with
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(* If this is a binop, find its precedence. *)
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| Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
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let token_prec = precedence c in
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(* If this is a binop that binds at least as tightly as the current binop,
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* consume it, otherwise we are done. *)
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if token_prec < expr_prec then lhs else begin
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(* Eat the binop. *)
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Stream.junk stream;
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(* Parse the primary expression after the binary operator. *)
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let rhs = parse_primary stream in
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(* Okay, we know this is a binop. *)
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let rhs =
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match Stream.peek stream with
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| Some (Token.Kwd c2) ->
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(* If BinOp binds less tightly with rhs than the operator after
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* rhs, let the pending operator take rhs as its lhs. *)
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let next_prec = precedence c2 in
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if token_prec < next_prec
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then parse_bin_rhs (token_prec + 1) rhs stream
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else rhs
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| _ -> rhs
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in
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(* Merge lhs/rhs. *)
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let lhs = Ast.Binary (c, lhs, rhs) in
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parse_bin_rhs expr_prec lhs stream
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end
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| _ -> lhs
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(* expression
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* ::= primary binoprhs *)
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and parse_expr = parser
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| [< lhs=parse_primary; stream >] -> parse_bin_rhs 0 lhs stream
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(* prototype
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* ::= id '(' id* ')' *)
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let parse_prototype =
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let rec parse_args accumulator = parser
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| [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
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| [< >] -> accumulator
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in
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parser
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| [< 'Token.Ident id;
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'Token.Kwd '(' ?? "expected '(' in prototype";
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args=parse_args [];
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'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
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(* success. *)
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Ast.Prototype (id, Array.of_list (List.rev args))
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| [< >] ->
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raise (Stream.Error "expected function name in prototype")
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(* definition ::= 'def' prototype expression *)
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let parse_definition = parser
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| [< 'Token.Def; p=parse_prototype; e=parse_expr >] ->
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Ast.Function (p, e)
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(* toplevelexpr ::= expression *)
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let parse_toplevel = parser
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| [< e=parse_expr >] ->
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(* Make an anonymous proto. *)
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Ast.Function (Ast.Prototype ("", [||]), e)
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(* external ::= 'extern' prototype *)
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let parse_extern = parser
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| [< 'Token.Extern; e=parse_prototype >] -> e
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