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diff --git a/docs/tutorial/OCamlLangImpl2.rst b/docs/tutorial/OCamlLangImpl2.rst new file mode 100644 index 0000000000..07490e1f67 --- /dev/null +++ b/docs/tutorial/OCamlLangImpl2.rst @@ -0,0 +1,899 @@ +=========================================== +Kaleidoscope: Implementing a Parser and AST +=========================================== + +.. contents:: + :local: + +Written by `Chris Lattner <mailto:sabre@nondot.org>`_ and `Erick +Tryzelaar <mailto:idadesub@users.sourceforge.net>`_ + +Chapter 2 Introduction +====================== + +Welcome to Chapter 2 of the "`Implementing a language with LLVM in +Objective Caml <index.html>`_" tutorial. This chapter shows you how to +use the lexer, built in `Chapter 1 <OCamlLangImpl1.html>`_, to build a +full `parser <http://en.wikipedia.org/wiki/Parsing>`_ for our +Kaleidoscope language. Once we have a parser, we'll define and build an +`Abstract Syntax +Tree <http://en.wikipedia.org/wiki/Abstract_syntax_tree>`_ (AST). + +The parser we will build uses a combination of `Recursive Descent +Parsing <http://en.wikipedia.org/wiki/Recursive_descent_parser>`_ and +`Operator-Precedence +Parsing <http://en.wikipedia.org/wiki/Operator-precedence_parser>`_ to +parse the Kaleidoscope language (the latter for binary expressions and +the former for everything else). Before we get to parsing though, lets +talk about the output of the parser: the Abstract Syntax Tree. + +The Abstract Syntax Tree (AST) +============================== + +The AST for a program captures its behavior in such a way that it is +easy for later stages of the compiler (e.g. code generation) to +interpret. We basically want one object for each construct in the +language, and the AST should closely model the language. In +Kaleidoscope, we have expressions, a prototype, and a function object. +We'll start with expressions first: + +.. code-block:: ocaml + + (* expr - Base type for all expression nodes. *) + type expr = + (* variant for numeric literals like "1.0". *) + | Number of float + +The code above shows the definition of the base ExprAST class and one +subclass which we use for numeric literals. The important thing to note +about this code is that the Number variant captures the numeric value of +the literal as an instance variable. This allows later phases of the +compiler to know what the stored numeric value is. + +Right now we only create the AST, so there are no useful functions on +them. It would be very easy to add a function to pretty print the code, +for example. Here are the other expression AST node definitions that +we'll use in the basic form of the Kaleidoscope language: + +.. code-block:: ocaml + + (* variant for referencing a variable, like "a". *) + | Variable of string + + (* variant for a binary operator. *) + | Binary of char * expr * expr + + (* variant for function calls. *) + | Call of string * expr array + +This is all (intentionally) rather straight-forward: variables capture +the variable name, binary operators capture their opcode (e.g. '+'), and +calls capture a function name as well as a list of any argument +expressions. One thing that is nice about our AST is that it captures +the language features without talking about the syntax of the language. +Note that there is no discussion about precedence of binary operators, +lexical structure, etc. + +For our basic language, these are all of the expression nodes we'll +define. Because it doesn't have conditional control flow, it isn't +Turing-complete; we'll fix that in a later installment. The two things +we need next are a way to talk about the interface to a function, and a +way to talk about functions themselves: + +.. code-block:: ocaml + + (* proto - This type represents the "prototype" for a function, which captures + * its name, and its argument names (thus implicitly the number of arguments the + * function takes). *) + type proto = Prototype of string * string array + + (* func - This type represents a function definition itself. *) + type func = Function of proto * expr + +In Kaleidoscope, functions are typed with just a count of their +arguments. Since all values are double precision floating point, the +type of each argument doesn't need to be stored anywhere. In a more +aggressive and realistic language, the "expr" variants would probably +have a type field. + +With this scaffolding, we can now talk about parsing expressions and +function bodies in Kaleidoscope. + +Parser Basics +============= + +Now that we have an AST to build, we need to define the parser code to +build it. The idea here is that we want to parse something like "x+y" +(which is returned as three tokens by the lexer) into an AST that could +be generated with calls like this: + +.. code-block:: ocaml + + let x = Variable "x" in + let y = Variable "y" in + let result = Binary ('+', x, y) in + ... + +The error handling routines make use of the builtin ``Stream.Failure`` +and ``Stream.Error``s. ``Stream.Failure`` is raised when the parser is +unable to find any matching token in the first position of a pattern. +``Stream.Error`` is raised when the first token matches, but the rest do +not. The error recovery in our parser will not be the best and is not +particular user-friendly, but it will be enough for our tutorial. These +exceptions make it easier to handle errors in routines that have various +return types. + +With these basic types and exceptions, we can implement the first piece +of our grammar: numeric literals. + +Basic Expression Parsing +======================== + +We start with numeric literals, because they are the simplest to +process. For each production in our grammar, we'll define a function +which parses that production. We call this class of expressions +"primary" expressions, for reasons that will become more clear `later in +the tutorial <OCamlLangImpl6.html#unary>`_. In order to parse an +arbitrary primary expression, we need to determine what sort of +expression it is. For numeric literals, we have: + +.. code-block:: ocaml + + (* primary + * ::= identifier + * ::= numberexpr + * ::= parenexpr *) + parse_primary = parser + (* numberexpr ::= number *) + | [< 'Token.Number n >] -> Ast.Number n + +This routine is very simple: it expects to be called when the current +token is a ``Token.Number`` token. It takes the current number value, +creates a ``Ast.Number`` node, advances the lexer to the next token, and +finally returns. + +There are some interesting aspects to this. The most important one is +that this routine eats all of the tokens that correspond to the +production and returns the lexer buffer with the next token (which is +not part of the grammar production) ready to go. This is a fairly +standard way to go for recursive descent parsers. For a better example, +the parenthesis operator is defined like this: + +.. code-block:: ocaml + + (* parenexpr ::= '(' expression ')' *) + | [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e + +This function illustrates a number of interesting things about the +parser: + +1) It shows how we use the ``Stream.Error`` exception. When called, this +function expects that the current token is a '(' token, but after +parsing the subexpression, it is possible that there is no ')' waiting. +For example, if the user types in "(4 x" instead of "(4)", the parser +should emit an error. Because errors can occur, the parser needs a way +to indicate that they happened. In our parser, we use the camlp4 +shortcut syntax ``token ?? "parse error"``, where if the token before +the ``??`` does not match, then ``Stream.Error "parse error"`` will be +raised. + +2) Another interesting aspect of this function is that it uses recursion +by calling ``Parser.parse_primary`` (we will soon see that +``Parser.parse_primary`` can call ``Parser.parse_primary``). This is +powerful because it allows us to handle recursive grammars, and keeps +each production very simple. Note that parentheses do not cause +construction of AST nodes themselves. While we could do it this way, the +most important role of parentheses are to guide the parser and provide +grouping. Once the parser constructs the AST, parentheses are not +needed. + +The next simple production is for handling variable references and +function calls: + +.. code-block:: ocaml + + (* identifierexpr + * ::= identifier + * ::= identifier '(' argumentexpr ')' *) + | [< 'Token.Ident id; stream >] -> + let rec parse_args accumulator = parser + | [< e=parse_expr; stream >] -> + begin parser + | [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e + | [< >] -> e :: accumulator + end stream + | [< >] -> accumulator + in + let rec parse_ident id = parser + (* Call. *) + | [< 'Token.Kwd '('; + args=parse_args []; + 'Token.Kwd ')' ?? "expected ')'">] -> + Ast.Call (id, Array.of_list (List.rev args)) + + (* Simple variable ref. *) + | [< >] -> Ast.Variable id + in + parse_ident id stream + +This routine follows the same style as the other routines. (It expects +to be called if the current token is a ``Token.Ident`` token). It also +has recursion and error handling. One interesting aspect of this is that +it uses *look-ahead* to determine if the current identifier is a stand +alone variable reference or if it is a function call expression. It +handles this by checking to see if the token after the identifier is a +'(' token, constructing either a ``Ast.Variable`` or ``Ast.Call`` node +as appropriate. + +We finish up by raising an exception if we received a token we didn't +expect: + +.. code-block:: ocaml + + | [< >] -> raise (Stream.Error "unknown token when expecting an expression.") + +Now that basic expressions are handled, we need to handle binary +expressions. They are a bit more complex. + +Binary Expression Parsing +========================= + +Binary expressions are significantly harder to parse because they are +often ambiguous. For example, when given the string "x+y\*z", the parser +can choose to parse it as either "(x+y)\*z" or "x+(y\*z)". With common +definitions from mathematics, we expect the later parse, because "\*" +(multiplication) has higher *precedence* than "+" (addition). + +There are many ways to handle this, but an elegant and efficient way is +to use `Operator-Precedence +Parsing <http://en.wikipedia.org/wiki/Operator-precedence_parser>`_. +This parsing technique uses the precedence of binary operators to guide +recursion. To start with, we need a table of precedences: + +.. code-block:: ocaml + + (* binop_precedence - This holds the precedence for each binary operator that is + * defined *) + let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10 + + (* precedence - Get the precedence of the pending binary operator token. *) + let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1 + + ... + + let main () = + (* Install standard binary operators. + * 1 is the lowest precedence. *) + Hashtbl.add Parser.binop_precedence '<' 10; + Hashtbl.add Parser.binop_precedence '+' 20; + Hashtbl.add Parser.binop_precedence '-' 20; + Hashtbl.add Parser.binop_precedence '*' 40; (* highest. *) + ... + +For the basic form of Kaleidoscope, we will only support 4 binary +operators (this can obviously be extended by you, our brave and intrepid +reader). The ``Parser.precedence`` function returns the precedence for +the current token, or -1 if the token is not a binary operator. Having a +``Hashtbl.t`` makes it easy to add new operators and makes it clear that +the algorithm doesn't depend on the specific operators involved, but it +would be easy enough to eliminate the ``Hashtbl.t`` and do the +comparisons in the ``Parser.precedence`` function. (Or just use a +fixed-size array). + +With the helper above defined, we can now start parsing binary +expressions. The basic idea of operator precedence parsing is to break +down an expression with potentially ambiguous binary operators into +pieces. Consider ,for example, the expression "a+b+(c+d)\*e\*f+g". +Operator precedence parsing considers this as a stream of primary +expressions separated by binary operators. As such, it will first parse +the leading primary expression "a", then it will see the pairs [+, b] +[+, (c+d)] [\*, e] [\*, f] and [+, g]. Note that because parentheses are +primary expressions, the binary expression parser doesn't need to worry +about nested subexpressions like (c+d) at all. + +To start, an expression is a primary expression potentially followed by +a sequence of [binop,primaryexpr] pairs: + +.. code-block:: ocaml + + (* expression + * ::= primary binoprhs *) + and parse_expr = parser + | [< lhs=parse_primary; stream >] -> parse_bin_rhs 0 lhs stream + +``Parser.parse_bin_rhs`` is the function that parses the sequence of +pairs for us. It takes a precedence and a pointer to an expression for +the part that has been parsed so far. Note that "x" is a perfectly valid +expression: As such, "binoprhs" is allowed to be empty, in which case it +returns the expression that is passed into it. In our example above, the +code passes the expression for "a" into ``Parser.parse_bin_rhs`` and the +current token is "+". + +The precedence value passed into ``Parser.parse_bin_rhs`` indicates the +*minimal operator precedence* that the function is allowed to eat. For +example, if the current pair stream is [+, x] and +``Parser.parse_bin_rhs`` is passed in a precedence of 40, it will not +consume any tokens (because the precedence of '+' is only 20). With this +in mind, ``Parser.parse_bin_rhs`` starts with: + +.. code-block:: ocaml + + (* binoprhs + * ::= ('+' primary)* *) + and parse_bin_rhs expr_prec lhs stream = + match Stream.peek stream with + (* If this is a binop, find its precedence. *) + | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c -> + let token_prec = precedence c in + + (* If this is a binop that binds at least as tightly as the current binop, + * consume it, otherwise we are done. *) + if token_prec < expr_prec then lhs else begin + +This code gets the precedence of the current token and checks to see if +if is too low. Because we defined invalid tokens to have a precedence of +-1, this check implicitly knows that the pair-stream ends when the token +stream runs out of binary operators. If this check succeeds, we know +that the token is a binary operator and that it will be included in this +expression: + +.. code-block:: ocaml + + (* Eat the binop. *) + Stream.junk stream; + + (* Okay, we know this is a binop. *) + let rhs = + match Stream.peek stream with + | Some (Token.Kwd c2) -> + +As such, this code eats (and remembers) the binary operator and then +parses the primary expression that follows. This builds up the whole +pair, the first of which is [+, b] for the running example. + +Now that we parsed the left-hand side of an expression and one pair of +the RHS sequence, we have to decide which way the expression associates. +In particular, we could have "(a+b) binop unparsed" or "a + (b binop +unparsed)". To determine this, we look ahead at "binop" to determine its +precedence and compare it to BinOp's precedence (which is '+' in this +case): + +.. code-block:: ocaml + + (* If BinOp binds less tightly with rhs than the operator after + * rhs, let the pending operator take rhs as its lhs. *) + let next_prec = precedence c2 in + if token_prec < next_prec + +If the precedence of the binop to the right of "RHS" is lower or equal +to the precedence of our current operator, then we know that the +parentheses associate as "(a+b) binop ...". In our example, the current +operator is "+" and the next operator is "+", we know that they have the +same precedence. In this case we'll create the AST node for "a+b", and +then continue parsing: + +.. code-block:: ocaml + + ... if body omitted ... + in + + (* Merge lhs/rhs. *) + let lhs = Ast.Binary (c, lhs, rhs) in + parse_bin_rhs expr_prec lhs stream + end + +In our example above, this will turn "a+b+" into "(a+b)" and execute the +next iteration of the loop, with "+" as the current token. The code +above will eat, remember, and parse "(c+d)" as the primary expression, +which makes the current pair equal to [+, (c+d)]. It will then evaluate +the 'if' conditional above with "\*" as the binop to the right of the +primary. In this case, the precedence of "\*" is higher than the +precedence of "+" so the if condition will be entered. + +The critical question left here is "how can the if condition parse the +right hand side in full"? In particular, to build the AST correctly for +our example, it needs to get all of "(c+d)\*e\*f" as the RHS expression +variable. The code to do this is surprisingly simple (code from the +above two blocks duplicated for context): + +.. code-block:: ocaml + + match Stream.peek stream with + | Some (Token.Kwd c2) -> + (* If BinOp binds less tightly with rhs than the operator after + * rhs, let the pending operator take rhs as its lhs. *) + if token_prec < precedence c2 + then parse_bin_rhs (token_prec + 1) rhs stream + else rhs + | _ -> rhs + in + + (* Merge lhs/rhs. *) + let lhs = Ast.Binary (c, lhs, rhs) in + parse_bin_rhs expr_prec lhs stream + end + +At this point, we know that the binary operator to the RHS of our +primary has higher precedence than the binop we are currently parsing. +As such, we know that any sequence of pairs whose operators are all +higher precedence than "+" should be parsed together and returned as +"RHS". To do this, we recursively invoke the ``Parser.parse_bin_rhs`` +function specifying "token\_prec+1" as the minimum precedence required +for it to continue. In our example above, this will cause it to return +the AST node for "(c+d)\*e\*f" as RHS, which is then set as the RHS of +the '+' expression. + +Finally, on the next iteration of the while loop, the "+g" piece is +parsed and added to the AST. With this little bit of code (14 +non-trivial lines), we correctly handle fully general binary expression +parsing in a very elegant way. This was a whirlwind tour of this code, +and it is somewhat subtle. I recommend running through it with a few +tough examples to see how it works. + +This wraps up handling of expressions. At this point, we can point the +parser at an arbitrary token stream and build an expression from it, +stopping at the first token that is not part of the expression. Next up +we need to handle function definitions, etc. + +Parsing the Rest +================ + +The next thing missing is handling of function prototypes. In +Kaleidoscope, these are used both for 'extern' function declarations as +well as function body definitions. The code to do this is +straight-forward and not very interesting (once you've survived +expressions): + +.. code-block:: ocaml + + (* prototype + * ::= id '(' id* ')' *) + let parse_prototype = + let rec parse_args accumulator = parser + | [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e + | [< >] -> accumulator + in + + parser + | [< 'Token.Ident id; + 'Token.Kwd '(' ?? "expected '(' in prototype"; + args=parse_args []; + 'Token.Kwd ')' ?? "expected ')' in prototype" >] -> + (* success. *) + Ast.Prototype (id, Array.of_list (List.rev args)) + + | [< >] -> + raise (Stream.Error "expected function name in prototype") + +Given this, a function definition is very simple, just a prototype plus +an expression to implement the body: + +.. code-block:: ocaml + + (* definition ::= 'def' prototype expression *) + let parse_definition = parser + | [< 'Token.Def; p=parse_prototype; e=parse_expr >] -> + Ast.Function (p, e) + +In addition, we support 'extern' to declare functions like 'sin' and +'cos' as well as to support forward declaration of user functions. These +'extern's are just prototypes with no body: + +.. code-block:: ocaml + + (* external ::= 'extern' prototype *) + let parse_extern = parser + | [< 'Token.Extern; e=parse_prototype >] -> e + +Finally, we'll also let the user type in arbitrary top-level expressions +and evaluate them on the fly. We will handle this by defining anonymous +nullary (zero argument) functions for them: + +.. code-block:: ocaml + + (* toplevelexpr ::= expression *) + let parse_toplevel = parser + | [< e=parse_expr >] -> + (* Make an anonymous proto. *) + Ast.Function (Ast.Prototype ("", [||]), e) + +Now that we have all the pieces, let's build a little driver that will +let us actually *execute* this code we've built! + +The Driver +========== + +The driver for this simply invokes all of the parsing pieces with a +top-level dispatch loop. There isn't much interesting here, so I'll just +include the top-level loop. See `below <#code>`_ for full code in the +"Top-Level Parsing" section. + +.. code-block:: ocaml + + (* top ::= definition | external | expression | ';' *) + let rec main_loop stream = + match Stream.peek stream with + | None -> () + + (* ignore top-level semicolons. *) + | Some (Token.Kwd ';') -> + Stream.junk stream; + main_loop stream + + | Some token -> + begin + try match token with + | Token.Def -> + ignore(Parser.parse_definition stream); + print_endline "parsed a function definition."; + | Token.Extern -> + ignore(Parser.parse_extern stream); + print_endline "parsed an extern."; + | _ -> + (* Evaluate a top-level expression into an anonymous function. *) + ignore(Parser.parse_toplevel stream); + print_endline "parsed a top-level expr"; + with Stream.Error s -> + (* Skip token for error recovery. *) + Stream.junk stream; + print_endline s; + end; + print_string "ready> "; flush stdout; + main_loop stream + +The most interesting part of this is that we ignore top-level +semicolons. Why is this, you ask? The basic reason is that if you type +"4 + 5" at the command line, the parser doesn't know whether that is the +end of what you will type or not. For example, on the next line you +could type "def foo..." in which case 4+5 is the end of a top-level +expression. Alternatively you could type "\* 6", which would continue +the expression. Having top-level semicolons allows you to type "4+5;", +and the parser will know you are done. + +Conclusions +=========== + +With just under 300 lines of commented code (240 lines of non-comment, +non-blank code), we fully defined our minimal language, including a +lexer, parser, and AST builder. With this done, the executable will +validate Kaleidoscope code and tell us if it is grammatically invalid. +For example, here is a sample interaction: + +.. code-block:: bash + + $ ./toy.byte + ready> def foo(x y) x+foo(y, 4.0); + Parsed a function definition. + ready> def foo(x y) x+y y; + Parsed a function definition. + Parsed a top-level expr + ready> def foo(x y) x+y ); + Parsed a function definition. + Error: unknown token when expecting an expression + ready> extern sin(a); + ready> Parsed an extern + ready> ^D + $ + +There is a lot of room for extension here. You can define new AST nodes, +extend the language in many ways, etc. In the `next +installment <OCamlLangImpl3.html>`_, we will describe how to generate +LLVM Intermediate Representation (IR) from the AST. + +Full Code Listing +================= + +Here is the complete code listing for this and the previous chapter. +Note that it is fully self-contained: you don't need LLVM or any +external libraries at all for this. (Besides the ocaml standard +libraries, of course.) To build this, just compile with: + +.. code-block:: bash + + # Compile + ocamlbuild toy.byte + # Run + ./toy.byte + +Here is the code: + +\_tags: + :: + + <{lexer,parser}.ml>: use_camlp4, pp(camlp4of) + +token.ml: + .. code-block:: ocaml + + (*===----------------------------------------------------------------------=== + * Lexer Tokens + *===----------------------------------------------------------------------===*) + + (* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of + * these others for known things. *) + type token = + (* commands *) + | Def | Extern + + (* primary *) + | Ident of string | Number of float + + (* unknown *) + | Kwd of char + +lexer.ml: + .. code-block:: ocaml + + (*===----------------------------------------------------------------------=== + * Lexer + *===----------------------------------------------------------------------===*) + + let rec lex = parser + (* Skip any whitespace. *) + | [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream + + (* identifier: [a-zA-Z][a-zA-Z0-9] *) + | [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] -> + let buffer = Buffer.create 1 in + Buffer.add_char buffer c; + lex_ident buffer stream + + (* number: [0-9.]+ *) + | [< ' ('0' .. '9' as c); stream >] -> + let buffer = Buffer.create 1 in + Buffer.add_char buffer c; + lex_number buffer stream + + (* Comment until end of line. *) + | [< ' ('#'); stream >] -> + lex_comment stream + + (* Otherwise, just return the character as its ascii value. *) + | [< 'c; stream >] -> + [< 'Token.Kwd c; lex stream >] + + (* end of stream. *) + | [< >] -> [< >] + + and lex_number buffer = parser + | [< ' ('0' .. '9' | '.' as c); stream >] -> + Buffer.add_char buffer c; + lex_number buffer stream + | [< stream=lex >] -> + [< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >] + + and lex_ident buffer = parser + | [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] -> + Buffer.add_char buffer c; + lex_ident buffer stream + | [< stream=lex >] -> + match Buffer.contents buffer with + | "def" -> [< 'Token.Def; stream >] + | "extern" -> [< 'Token.Extern; stream >] + | id -> [< 'Token.Ident id; stream >] + + and lex_comment = parser + | [< ' ('\n'); stream=lex >] -> stream + | [< 'c; e=lex_comment >] -> e + | [< >] -> [< >] + +ast.ml: + .. code-block:: ocaml + + (*===----------------------------------------------------------------------=== + * Abstract Syntax Tree (aka Parse Tree) + *===----------------------------------------------------------------------===*) + + (* expr - Base type for all expression nodes. *) + type expr = + (* variant for numeric literals like "1.0". *) + | Number of float + + (* variant for referencing a variable, like "a". *) + | Variable of string + + (* variant for a binary operator. *) + | Binary of char * expr * expr + + (* variant for function calls. *) + | Call of string * expr array + + (* proto - This type represents the "prototype" for a function, which captures + * its name, and its argument names (thus implicitly the number of arguments the + * function takes). *) + type proto = Prototype of string * string array + + (* func - This type represents a function definition itself. *) + type func = Function of proto * expr + +parser.ml: + .. code-block:: ocaml + + (*===---------------------------------------------------------------------=== + * Parser + *===---------------------------------------------------------------------===*) + + (* binop_precedence - This holds the precedence for each binary operator that is + * defined *) + let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10 + + (* precedence - Get the precedence of the pending binary operator token. *) + let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1 + + (* primary + * ::= identifier + * ::= numberexpr + * ::= parenexpr *) + let rec parse_primary = parser + (* numberexpr ::= number *) + | [< 'Token.Number n >] -> Ast.Number n + + (* parenexpr ::= '(' expression ')' *) + | [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e + + (* identifierexpr + * ::= identifier + * ::= identifier '(' argumentexpr ')' *) + | [< 'Token.Ident id; stream >] -> + let rec parse_args accumulator = parser + | [< e=parse_expr; stream >] -> + begin parser + | [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e + | [< >] -> e :: accumulator + end stream + | [< >] -> accumulator + in + let rec parse_ident id = parser + (* Call. *) + | [< 'Token.Kwd '('; + args=parse_args []; + 'Token.Kwd ')' ?? "expected ')'">] -> + Ast.Call (id, Array.of_list (List.rev args)) + + (* Simple variable ref. *) + | [< >] -> Ast.Variable id + in + parse_ident id stream + + | [< >] -> raise (Stream.Error "unknown token when expecting an expression.") + + (* binoprhs + * ::= ('+' primary)* *) + and parse_bin_rhs expr_prec lhs stream = + match Stream.peek stream with + (* If this is a binop, find its precedence. *) + | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c -> + let token_prec = precedence c in + + (* If this is a binop that binds at least as tightly as the current binop, + * consume it, otherwise we are done. *) + if token_prec < expr_prec then lhs else begin + (* Eat the binop. *) + Stream.junk stream; + + (* Parse the primary expression after the binary operator. *) + let rhs = parse_primary stream in + + (* Okay, we know this is a binop. *) + let rhs = + match Stream.peek stream with + | Some (Token.Kwd c2) -> + (* If BinOp binds less tightly with rhs than the operator after + * rhs, let the pending operator take rhs as its lhs. *) + let next_prec = precedence c2 in + if token_prec < next_prec + then parse_bin_rhs (token_prec + 1) rhs stream + else rhs + | _ -> rhs + in + + (* Merge lhs/rhs. *) + let lhs = Ast.Binary (c, lhs, rhs) in + parse_bin_rhs expr_prec lhs stream + end + | _ -> lhs + + (* expression + * ::= primary binoprhs *) + and parse_expr = parser + | [< lhs=parse_primary; stream >] -> parse_bin_rhs 0 lhs stream + + (* prototype + * ::= id '(' id* ')' *) + let parse_prototype = + let rec parse_args accumulator = parser + | [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e + | [< >] -> accumulator + in + + parser + | [< 'Token.Ident id; + 'Token.Kwd '(' ?? "expected '(' in prototype"; + args=parse_args []; + 'Token.Kwd ')' ?? "expected ')' in prototype" >] -> + (* success. *) + Ast.Prototype (id, Array.of_list (List.rev args)) + + | [< >] -> + raise (Stream.Error "expected function name in prototype") + + (* definition ::= 'def' prototype expression *) + let parse_definition = parser + | [< 'Token.Def; p=parse_prototype; e=parse_expr >] -> + Ast.Function (p, e) + + (* toplevelexpr ::= expression *) + let parse_toplevel = parser + | [< e=parse_expr >] -> + (* Make an anonymous proto. *) + Ast.Function (Ast.Prototype ("", [||]), e) + + (* external ::= 'extern' prototype *) + let parse_extern = parser + | [< 'Token.Extern; e=parse_prototype >] -> e + +toplevel.ml: + .. code-block:: ocaml + + (*===----------------------------------------------------------------------=== + * Top-Level parsing and JIT Driver + *===----------------------------------------------------------------------===*) + + (* top ::= definition | external | expression | ';' *) + let rec main_loop stream = + match Stream.peek stream with + | None -> () + + (* ignore top-level semicolons. *) + | Some (Token.Kwd ';') -> + Stream.junk stream; + main_loop stream + + | Some token -> + begin + try match token with + | Token.Def -> + ignore(Parser.parse_definition stream); + print_endline "parsed a function definition."; + | Token.Extern -> + ignore(Parser.parse_extern stream); + print_endline "parsed an extern."; + | _ -> + (* Evaluate a top-level expression into an anonymous function. *) + ignore(Parser.parse_toplevel stream); + print_endline "parsed a top-level expr"; + with Stream.Error s -> + (* Skip token for error recovery. *) + Stream.junk stream; + print_endline s; + end; + print_string "ready> "; flush stdout; + main_loop stream + +toy.ml: + .. code-block:: ocaml + + (*===----------------------------------------------------------------------=== + * Main driver code. + *===----------------------------------------------------------------------===*) + + let main () = + (* Install standard binary operators. + * 1 is the lowest precedence. *) + Hashtbl.add Parser.binop_precedence '<' 10; + Hashtbl.add Parser.binop_precedence '+' 20; + Hashtbl.add Parser.binop_precedence '-' 20; + Hashtbl.add Parser.binop_precedence '*' 40; (* highest. *) + + (* Prime the first token. *) + print_string "ready> "; flush stdout; + let stream = Lexer.lex (Stream.of_channel stdin) in + + (* Run the main "interpreter loop" now. *) + Toplevel.main_loop stream; + ;; + + main () + +`Next: Implementing Code Generation to LLVM IR <OCamlLangImpl3.html>`_ + |