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49 with LLVM in Objective Caml</a>" tutorial. This chapter shows you how to use
52 Kaleidoscope language. Once we have a parser, we'll define and build an <a
60 Parsing</a> to parse the Kaleidoscope language (the latter for
61 binary expressions and the former for everything else). Before we get to
62 parsing though, lets talk about the output of the parser: the Abstract Syntax
63 Tree.</p>
65 </div>
67 <!-- *********************************************************************** -->
69 <!-- *********************************************************************** -->
73 <p>The AST for a program captures its behavior in such a way that it is easy for
74 later stages of the compiler (e.g. code generation) to interpret. We basically
75 want one object for each construct in the language, and the AST should closely
76 model the language. In Kaleidoscope, we have expressions, a prototype, and a
77 function object. We'll start with expressions first:</p>
80 <pre>
81 (* expr - Base type for all expression nodes. *)
82 type expr =
83 (* variant for numeric literals like "1.0". *)
84 | Number of float
85 </pre>
86 </div>
88 <p>The code above shows the definition of the base ExprAST class and one
89 subclass which we use for numeric literals. The important thing to note about
90 this code is that the Number variant captures the numeric value of the
91 literal as an instance variable. This allows later phases of the compiler to
92 know what the stored numeric value is.</p>
94 <p>Right now we only create the AST, so there are no useful functions on
95 them. It would be very easy to add a function to pretty print the code,
96 for example. Here are the other expression AST node definitions that we'll use
97 in the basic form of the Kaleidoscope language:
98 </p>
101 <pre>
102 (* variant for referencing a variable, like "a". *)
103 | Variable of string
105 (* variant for a binary operator. *)
106 | Binary of char * expr * expr
108 (* variant for function calls. *)
109 | Call of string * expr array
110 </pre>
111 </div>
113 <p>This is all (intentionally) rather straight-forward: variables capture the
114 variable name, binary operators capture their opcode (e.g. '+'), and calls
115 capture a function name as well as a list of any argument expressions. One thing
116 that is nice about our AST is that it captures the language features without
117 talking about the syntax of the language. Note that there is no discussion about
118 precedence of binary operators, lexical structure, etc.</p>
120 <p>For our basic language, these are all of the expression nodes we'll define.
121 Because it doesn't have conditional control flow, it isn't Turing-complete;
122 we'll fix that in a later installment. The two things we need next are a way
123 to talk about the interface to a function, and a way to talk about functions
124 themselves:</p>
127 <pre>
128 (* proto - This type represents the "prototype" for a function, which captures
129 * its name, and its argument names (thus implicitly the number of arguments the
130 * function takes). *)
131 type proto = Prototype of string * string array
133 (* func - This type represents a function definition itself. *)
134 type func = Function of proto * expr
135 </pre>
136 </div>
138 <p>In Kaleidoscope, functions are typed with just a count of their arguments.
139 Since all values are double precision floating point, the type of each argument
140 doesn't need to be stored anywhere. In a more aggressive and realistic
143 <p>With this scaffolding, we can now talk about parsing expressions and function
144 bodies in Kaleidoscope.</p>
146 </div>
148 <!-- *********************************************************************** -->
150 <!-- *********************************************************************** -->
154 <p>Now that we have an AST to build, we need to define the parser code to build
155 it. The idea here is that we want to parse something like "x+y" (which is
156 returned as three tokens by the lexer) into an AST that could be generated with
157 calls like this:</p>
160 <pre>
161 let x = Variable "x" in
162 let y = Variable "y" in
163 let result = Binary ('+', x, y) in
164 ...
165 </pre>
166 </div>
168 <p>
171 unable to find any matching token in the first position of a pattern.
173 not. The error recovery in our parser will not be the best and is not
174 particular user-friendly, but it will be enough for our tutorial. These
175 exceptions make it easier to handle errors in routines that have various return
176 types.</p>
178 <p>With these basic types and exceptions, we can implement the first
179 piece of our grammar: numeric literals.</p>
181 </div>
183 <!-- *********************************************************************** -->
185 Parsing</a></div>
186 <!-- *********************************************************************** -->
190 <p>We start with numeric literals, because they are the simplest to process.
191 For each production in our grammar, we'll define a function which parses that
192 production. We call this class of expressions "primary" expressions, for
194 later in the tutorial</a>. In order to parse an arbitrary primary expression,
195 we need to determine what sort of expression it is. For numeric literals, we
196 have:</p>
199 <pre>
200 (* primary
201 * ::= identifier
202 * ::= numberexpr
203 * ::= parenexpr *)
204 parse_primary = parser
205 (* numberexpr ::= number *)
207 </pre>
208 </div>
210 <p>This routine is very simple: it expects to be called when the current token
213 returns.</p>
215 <p>There are some interesting aspects to this. The most important one is that
216 this routine eats all of the tokens that correspond to the production and
217 returns the lexer buffer with the next token (which is not part of the grammar
218 production) ready to go. This is a fairly standard way to go for recursive
219 descent parsers. For a better example, the parenthesis operator is defined like
220 this:</p>
223 <pre>
224 (* parenexpr ::= '(' expression ')' *)
226 </pre>
227 </div>
229 <p>This function illustrates a number of interesting things about the
230 parser:</p>
232 <p>
234 function expects that the current token is a '(' token, but after parsing the
235 subexpression, it is possible that there is no ')' waiting. For example, if
237 Because errors can occur, the parser needs a way to indicate that they
245 powerful because it allows us to handle recursive grammars, and keeps each
246 production very simple. Note that parentheses do not cause construction of AST
247 nodes themselves. While we could do it this way, the most important role of
248 parentheses are to guide the parser and provide grouping. Once the parser
249 constructs the AST, parentheses are not needed.</p>
251 <p>The next simple production is for handling variable references and function
252 calls:</p>
255 <pre>
256 (* identifierexpr
257 * ::= identifier
258 * ::= identifier '(' argumentexpr ')' *)
260 let rec parse_args accumulator = parser
262 begin parser
265 end stream
267 in
268 let rec parse_ident id = parser
269 (* Call. *)
270 | [< 'Token.Kwd '(';
271 args=parse_args [];
273 Ast.Call (id, Array.of_list (List.rev args))
275 (* Simple variable ref. *)
277 in
278 parse_ident id stream
279 </pre>
280 </div>
282 <p>This routine follows the same style as the other routines. (It expects to be
284 recursion and error handling. One interesting aspect of this is that it uses
286 variable reference or if it is a function call expression. It handles this by
287 checking to see if the token after the identifier is a '(' token, constructing
289 </p>
291 <p>We finish up by raising an exception if we received a token we didn't
292 expect:</p>
295 <pre>
297 </pre>
298 </div>
300 <p>Now that basic expressions are handled, we need to handle binary expressions.
301 They are a bit more complex.</p>
303 </div>
305 <!-- *********************************************************************** -->
307 Parsing</a></div>
308 <!-- *********************************************************************** -->
312 <p>Binary expressions are significantly harder to parse because they are often
313 ambiguous. For example, when given the string "x+y*z", the parser can choose
315 mathematics, we expect the later parse, because "*" (multiplication) has
318 <p>There are many ways to handle this, but an elegant and efficient way is to
319 use <a href=
321 Parsing</a>. This parsing technique uses the precedence of binary operators to
322 guide recursion. To start with, we need a table of precedences:</p>
325 <pre>
326 (* binop_precedence - This holds the precedence for each binary operator that is
327 * defined *)
328 let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
330 (* precedence - Get the precedence of the pending binary operator token. *)
333 ...
335 let main () =
336 (* Install standard binary operators.
337 * 1 is the lowest precedence. *)
339 Hashtbl.add Parser.binop_precedence '+' 20;
340 Hashtbl.add Parser.binop_precedence '-' 20;
341 Hashtbl.add Parser.binop_precedence '*' 40; (* highest. *)
342 ...
343 </pre>
344 </div>
347 (this can obviously be extended by you, our brave and intrepid reader). The
350 makes it easy to add new operators and makes it clear that the algorithm doesn't
351 depend on the specific operators involved, but it would be easy enough to
355 <p>With the helper above defined, we can now start parsing binary expressions.
356 The basic idea of operator precedence parsing is to break down an expression
357 with potentially ambiguous binary operators into pieces. Consider ,for example,
358 the expression "a+b+(c+d)*e*f+g". Operator precedence parsing considers this
359 as a stream of primary expressions separated by binary operators. As such,
360 it will first parse the leading primary expression "a", then it will see the
361 pairs [+, b] [+, (c+d)] [*, e] [*, f] and [+, g]. Note that because parentheses
362 are primary expressions, the binary expression parser doesn't need to worry
363 about nested subexpressions like (c+d) at all.
364 </p>
366 <p>
367 To start, an expression is a primary expression potentially followed by a
368 sequence of [binop,primaryexpr] pairs:</p>
371 <pre>
372 (* expression
373 * ::= primary binoprhs *)
374 and parse_expr = parser
376 </pre>
377 </div>
380 pairs for us. It takes a precedence and a pointer to an expression for the part
381 that has been parsed so far. Note that "x" is a perfectly valid expression: As
382 such, "binoprhs" is allowed to be empty, in which case it returns the expression
383 that is passed into it. In our example above, the code passes the expression for
389 is passed in a precedence of 40, it will not consume any tokens (because the
391 starts with:</p>
394 <pre>
395 (* binoprhs
396 * ::= ('+' primary)* *)
397 and parse_bin_rhs expr_prec lhs stream =
398 match Stream.peek stream with
399 (* If this is a binop, find its precedence. *)
400 | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
401 let token_prec = precedence c in
403 (* If this is a binop that binds at least as tightly as the current binop,
404 * consume it, otherwise we are done. *)
405 if token_prec < expr_prec then lhs else begin
406 </pre>
407 </div>
409 <p>This code gets the precedence of the current token and checks to see if if is
410 too low. Because we defined invalid tokens to have a precedence of -1, this
411 check implicitly knows that the pair-stream ends when the token stream runs out
412 of binary operators. If this check succeeds, we know that the token is a binary
413 operator and that it will be included in this expression:</p>
416 <pre>
417 (* Eat the binop. *)
418 Stream.junk stream;
420 (* Okay, we know this is a binop. *)
421 let rhs =
422 match Stream.peek stream with
423 | Some (Token.Kwd c2) ->
424 </pre>
425 </div>
427 <p>As such, this code eats (and remembers) the binary operator and then parses
428 the primary expression that follows. This builds up the whole pair, the first of
429 which is [+, b] for the running example.</p>
431 <p>Now that we parsed the left-hand side of an expression and one pair of the
432 RHS sequence, we have to decide which way the expression associates. In
434 To determine this, we look ahead at "binop" to determine its precedence and
435 compare it to BinOp's precedence (which is '+' in this case):</p>
438 <pre>
439 (* If BinOp binds less tightly with rhs than the operator after
440 * rhs, let the pending operator take rhs as its lhs. *)
441 let next_prec = precedence c2 in
442 if token_prec < next_prec
443 </pre>
444 </div>
447 precedence of our current operator, then we know that the parentheses associate
449 operator is "+", we know that they have the same precedence. In this case we'll
453 <pre>
454 ... if body omitted ...
455 in
457 (* Merge lhs/rhs. *)
458 let lhs = Ast.Binary (c, lhs, rhs) in
459 parse_bin_rhs expr_prec lhs stream
460 end
461 </pre>
462 </div>
465 iteration of the loop, with "+" as the current token. The code above will eat,
466 remember, and parse "(c+d)" as the primary expression, which makes the
467 current pair equal to [+, (c+d)]. It will then evaluate the 'if' conditional above with
472 hand side in full"? In particular, to build the AST correctly for our example,
473 it needs to get all of "(c+d)*e*f" as the RHS expression variable. The code to
474 do this is surprisingly simple (code from the above two blocks duplicated for
475 context):</p>
478 <pre>
479 match Stream.peek stream with
480 | Some (Token.Kwd c2) ->
481 (* If BinOp binds less tightly with rhs than the operator after
482 * rhs, let the pending operator take rhs as its lhs. *)
483 if token_prec < precedence c2
485 else rhs
486 | _ -> rhs
487 in
489 (* Merge lhs/rhs. *)
490 let lhs = Ast.Binary (c, lhs, rhs) in
491 parse_bin_rhs expr_prec lhs stream
492 end
493 </pre>
494 </div>
496 <p>At this point, we know that the binary operator to the RHS of our primary
497 has higher precedence than the binop we are currently parsing. As such, we know
498 that any sequence of pairs whose operators are all higher precedence than "+"
499 should be parsed together and returned as "RHS". To do this, we recursively
501 the minimum precedence required for it to continue. In our example above, this
502 will cause it to return the AST node for "(c+d)*e*f" as RHS, which is then set
503 as the RHS of the '+' expression.</p>
506 and added to the AST. With this little bit of code (14 non-trivial lines), we
507 correctly handle fully general binary expression parsing in a very elegant way.
508 This was a whirlwind tour of this code, and it is somewhat subtle. I recommend
509 running through it with a few tough examples to see how it works.
510 </p>
512 <p>This wraps up handling of expressions. At this point, we can point the
513 parser at an arbitrary token stream and build an expression from it, stopping
514 at the first token that is not part of the expression. Next up we need to
515 handle function definitions, etc.</p>
517 </div>
519 <!-- *********************************************************************** -->
521 <!-- *********************************************************************** -->
525 <p>
526 The next thing missing is handling of function prototypes. In Kaleidoscope,
527 these are used both for 'extern' function declarations as well as function body
528 definitions. The code to do this is straight-forward and not very interesting
529 (once you've survived expressions):
530 </p>
533 <pre>
534 (* prototype
535 * ::= id '(' id* ')' *)
536 let parse_prototype =
537 let rec parse_args accumulator = parser
540 in
542 parser
543 | [< 'Token.Ident id;
544 'Token.Kwd '(' ?? "expected '(' in prototype";
545 args=parse_args [];
547 (* success. *)
548 Ast.Prototype (id, Array.of_list (List.rev args))
551 raise (Stream.Error "expected function name in prototype")
552 </pre>
553 </div>
555 <p>Given this, a function definition is very simple, just a prototype plus
556 an expression to implement the body:</p>
559 <pre>
560 (* definition ::= 'def' prototype expression *)
561 let parse_definition = parser
563 Ast.Function (p, e)
564 </pre>
565 </div>
567 <p>In addition, we support 'extern' to declare functions like 'sin' and 'cos' as
568 well as to support forward declaration of user functions. These 'extern's are just
569 prototypes with no body:</p>
572 <pre>
573 (* external ::= 'extern' prototype *)
574 let parse_extern = parser
576 </pre>
577 </div>
579 <p>Finally, we'll also let the user type in arbitrary top-level expressions and
580 evaluate them on the fly. We will handle this by defining anonymous nullary
581 (zero argument) functions for them:</p>
584 <pre>
585 (* toplevelexpr ::= expression *)
586 let parse_toplevel = parser
588 (* Make an anonymous proto. *)
589 Ast.Function (Ast.Prototype ("", [||]), e)
590 </pre>
591 </div>
593 <p>Now that we have all the pieces, let's build a little driver that will let us
596 </div>
598 <!-- *********************************************************************** -->
600 <!-- *********************************************************************** -->
604 <p>The driver for this simply invokes all of the parsing pieces with a top-level
605 dispatch loop. There isn't much interesting here, so I'll just include the
610 <pre>
611 (* top ::= definition | external | expression | ';' *)
612 let rec main_loop stream =
613 match Stream.peek stream with
614 | None -> ()
616 (* ignore top-level semicolons. *)
617 | Some (Token.Kwd ';') ->
618 Stream.junk stream;
619 main_loop stream
621 | Some token ->
622 begin
623 try match token with
624 | Token.Def ->
625 ignore(Parser.parse_definition stream);
626 print_endline "parsed a function definition.";
627 | Token.Extern ->
628 ignore(Parser.parse_extern stream);
629 print_endline "parsed an extern.";
630 | _ ->
631 (* Evaluate a top-level expression into an anonymous function. *)
632 ignore(Parser.parse_toplevel stream);
633 print_endline "parsed a top-level expr";
634 with Stream.Error s ->
635 (* Skip token for error recovery. *)
636 Stream.junk stream;
637 print_endline s;
638 end;
639 print_string "ready> "; flush stdout;
640 main_loop stream
641 </pre>
642 </div>
644 <p>The most interesting part of this is that we ignore top-level semicolons.
645 Why is this, you ask? The basic reason is that if you type "4 + 5" at the
646 command line, the parser doesn't know whether that is the end of what you will type
647 or not. For example, on the next line you could type "def foo..." in which case
649 which would continue the expression. Having top-level semicolons allows you to
652 </div>
654 <!-- *********************************************************************** -->
656 <!-- *********************************************************************** -->
661 non-blank code), we fully defined our minimal language, including a lexer,
662 parser, and AST builder. With this done, the executable will validate
663 Kaleidoscope code and tell us if it is grammatically invalid. For
664 example, here is a sample interaction:</p>
667 <pre>
670 Parsed a function definition.
672 Parsed a function definition.
673 Parsed a top-level expr
675 Parsed a function definition.
676 Error: unknown token when expecting an expression
678 ready> Parsed an extern
680 $
681 </pre>
682 </div>
684 <p>There is a lot of room for extension here. You can define new AST nodes,
686 next installment</a>, we will describe how to generate LLVM Intermediate
687 Representation (IR) from the AST.</p>
689 </div>
691 <!-- *********************************************************************** -->
693 <!-- *********************************************************************** -->
697 <p>
698 Here is the complete code listing for this and the previous chapter.
699 Note that it is fully self-contained: you don't need LLVM or any external
700 libraries at all for this. (Besides the ocaml standard libraries, of
701 course.) To build this, just compile with:</p>
704 <pre>
705 # Compile
706 ocamlbuild toy.byte
707 # Run
708 ./toy.byte
709 </pre>
710 </div>
714 <dl>
717 <pre>
719 </pre>
720 </dd>
724 <pre>
725 (*===----------------------------------------------------------------------===
726 * Lexer Tokens
727 *===----------------------------------------------------------------------===*)
729 (* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
730 * these others for known things. *)
731 type token =
732 (* commands *)
733 | Def | Extern
735 (* primary *)
736 | Ident of string | Number of float
738 (* unknown *)
739 | Kwd of char
740 </pre>
741 </dd>
745 <pre>
746 (*===----------------------------------------------------------------------===
747 * Lexer
748 *===----------------------------------------------------------------------===*)
750 let rec lex = parser
751 (* Skip any whitespace. *)
754 (* identifier: [a-zA-Z][a-zA-Z0-9] *)
756 let buffer = Buffer.create 1 in
757 Buffer.add_char buffer c;
758 lex_ident buffer stream
762 let buffer = Buffer.create 1 in
763 Buffer.add_char buffer c;
764 lex_number buffer stream
766 (* Comment until end of line. *)
768 lex_comment stream
770 (* Otherwise, just return the character as its ascii value. *)
774 (* end of stream. *)
777 and lex_number buffer = parser
779 Buffer.add_char buffer c;
780 lex_number buffer stream
784 and lex_ident buffer = parser
786 Buffer.add_char buffer c;
787 lex_ident buffer stream
789 match Buffer.contents buffer with
794 and lex_comment = parser
798 </pre>
799 </dd>
803 <pre>
804 (*===----------------------------------------------------------------------===
805 * Abstract Syntax Tree (aka Parse Tree)
806 *===----------------------------------------------------------------------===*)
808 (* expr - Base type for all expression nodes. *)
809 type expr =
810 (* variant for numeric literals like "1.0". *)
811 | Number of float
813 (* variant for referencing a variable, like "a". *)
814 | Variable of string
816 (* variant for a binary operator. *)
817 | Binary of char * expr * expr
819 (* variant for function calls. *)
820 | Call of string * expr array
822 (* proto - This type represents the "prototype" for a function, which captures
823 * its name, and its argument names (thus implicitly the number of arguments the
824 * function takes). *)
825 type proto = Prototype of string * string array
827 (* func - This type represents a function definition itself. *)
828 type func = Function of proto * expr
829 </pre>
830 </dd>
834 <pre>
835 (*===---------------------------------------------------------------------===
836 * Parser
837 *===---------------------------------------------------------------------===*)
839 (* binop_precedence - This holds the precedence for each binary operator that is
840 * defined *)
841 let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
843 (* precedence - Get the precedence of the pending binary operator token. *)
846 (* primary
847 * ::= identifier
848 * ::= numberexpr
849 * ::= parenexpr *)
850 let rec parse_primary = parser
851 (* numberexpr ::= number *)
854 (* parenexpr ::= '(' expression ')' *)
857 (* identifierexpr
858 * ::= identifier
859 * ::= identifier '(' argumentexpr ')' *)
861 let rec parse_args accumulator = parser
863 begin parser
866 end stream
868 in
869 let rec parse_ident id = parser
870 (* Call. *)
871 | [< 'Token.Kwd '(';
872 args=parse_args [];
874 Ast.Call (id, Array.of_list (List.rev args))
876 (* Simple variable ref. *)
878 in
879 parse_ident id stream
883 (* binoprhs
884 * ::= ('+' primary)* *)
885 and parse_bin_rhs expr_prec lhs stream =
886 match Stream.peek stream with
887 (* If this is a binop, find its precedence. *)
888 | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
889 let token_prec = precedence c in
891 (* If this is a binop that binds at least as tightly as the current binop,
892 * consume it, otherwise we are done. *)
893 if token_prec < expr_prec then lhs else begin
894 (* Eat the binop. *)
895 Stream.junk stream;
897 (* Parse the primary expression after the binary operator. *)
898 let rhs = parse_primary stream in
900 (* Okay, we know this is a binop. *)
901 let rhs =
902 match Stream.peek stream with
903 | Some (Token.Kwd c2) ->
904 (* If BinOp binds less tightly with rhs than the operator after
905 * rhs, let the pending operator take rhs as its lhs. *)
906 let next_prec = precedence c2 in
907 if token_prec < next_prec
908 then parse_bin_rhs (token_prec + 1) rhs stream
909 else rhs
910 | _ -> rhs
911 in
913 (* Merge lhs/rhs. *)
914 let lhs = Ast.Binary (c, lhs, rhs) in
915 parse_bin_rhs expr_prec lhs stream
916 end
917 | _ -> lhs
919 (* expression
920 * ::= primary binoprhs *)
921 and parse_expr = parser
924 (* prototype
925 * ::= id '(' id* ')' *)
926 let parse_prototype =
927 let rec parse_args accumulator = parser
930 in
932 parser
933 | [< 'Token.Ident id;
934 'Token.Kwd '(' ?? "expected '(' in prototype";
935 args=parse_args [];
937 (* success. *)
938 Ast.Prototype (id, Array.of_list (List.rev args))
941 raise (Stream.Error "expected function name in prototype")
943 (* definition ::= 'def' prototype expression *)
944 let parse_definition = parser
946 Ast.Function (p, e)
948 (* toplevelexpr ::= expression *)
949 let parse_toplevel = parser
951 (* Make an anonymous proto. *)
952 Ast.Function (Ast.Prototype ("", [||]), e)
954 (* external ::= 'extern' prototype *)
955 let parse_extern = parser
957 </pre>
958 </dd>
962 <pre>
963 (*===----------------------------------------------------------------------===
964 * Top-Level parsing and JIT Driver
965 *===----------------------------------------------------------------------===*)
967 (* top ::= definition | external | expression | ';' *)
968 let rec main_loop stream =
969 match Stream.peek stream with
970 | None -> ()
972 (* ignore top-level semicolons. *)
973 | Some (Token.Kwd ';') ->
974 Stream.junk stream;
975 main_loop stream
977 | Some token ->
978 begin
979 try match token with
980 | Token.Def ->
981 ignore(Parser.parse_definition stream);
982 print_endline "parsed a function definition.";
983 | Token.Extern ->
984 ignore(Parser.parse_extern stream);
985 print_endline "parsed an extern.";
986 | _ ->
987 (* Evaluate a top-level expression into an anonymous function. *)
988 ignore(Parser.parse_toplevel stream);
989 print_endline "parsed a top-level expr";
990 with Stream.Error s ->
991 (* Skip token for error recovery. *)
992 Stream.junk stream;
993 print_endline s;
994 end;
995 print_string "ready> "; flush stdout;
996 main_loop stream
997 </pre>
998 </dd>
1002 <pre>
1003 (*===----------------------------------------------------------------------===
1004 * Main driver code.
1005 *===----------------------------------------------------------------------===*)
1007 let main () =
1008 (* Install standard binary operators.
1009 * 1 is the lowest precedence. *)
1011 Hashtbl.add Parser.binop_precedence '+' 20;
1012 Hashtbl.add Parser.binop_precedence '-' 20;
1013 Hashtbl.add Parser.binop_precedence '*' 40; (* highest. *)
1015 (* Prime the first token. *)
1016 print_string "ready> "; flush stdout;
1017 let stream = Lexer.lex (Stream.of_channel stdin) in
1019 (* Run the main "interpreter loop" now. *)
1020 Toplevel.main_loop stream;
1021 ;;
1023 main ()
1024 </pre>
1025 </dd>
1026 </dl>
1029 </div>
1031 <!-- *********************************************************************** -->
1032 <hr>
1033 <address>
1042 Last modified: $Date$
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1044 </body>
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