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- <title>Kaleidoscope: Implementing a Parser and AST</title>
- <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
- <meta name="author" content="Chris Lattner">
- <link rel="stylesheet" href="../_static/llvm.css" type="text/css">
-</head>
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-<body>
-
-<h1>Kaleidoscope: Implementing a Parser and AST</h1>
-
-<ul>
-<li><a href="index.html">Up to Tutorial Index</a></li>
-<li>Chapter 2
- <ol>
- <li><a href="#intro">Chapter 2 Introduction</a></li>
- <li><a href="#ast">The Abstract Syntax Tree (AST)</a></li>
- <li><a href="#parserbasics">Parser Basics</a></li>
- <li><a href="#parserprimexprs">Basic Expression Parsing</a></li>
- <li><a href="#parserbinops">Binary Expression Parsing</a></li>
- <li><a href="#parsertop">Parsing the Rest</a></li>
- <li><a href="#driver">The Driver</a></li>
- <li><a href="#conclusions">Conclusions</a></li>
- <li><a href="#code">Full Code Listing</a></li>
- </ol>
-</li>
-<li><a href="LangImpl3.html">Chapter 3</a>: Code generation to LLVM IR</li>
-</ul>
-
-<div class="doc_author">
- <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
-</div>
-
-<!-- *********************************************************************** -->
-<h2><a name="intro">Chapter 2 Introduction</a></h2>
-<!-- *********************************************************************** -->
-
-<div>
-
-<p>Welcome to Chapter 2 of the "<a href="index.html">Implementing a language
-with LLVM</a>" tutorial. This chapter shows you how to use the lexer, built in
-<a href="LangImpl1.html">Chapter 1</a>, to build a full <a
-href="http://en.wikipedia.org/wiki/Parsing">parser</a> for
-our Kaleidoscope language. Once we have a parser, we'll define and build an <a
-href="http://en.wikipedia.org/wiki/Abstract_syntax_tree">Abstract Syntax
-Tree</a> (AST).</p>
-
-<p>The parser we will build uses a combination of <a
-href="http://en.wikipedia.org/wiki/Recursive_descent_parser">Recursive Descent
-Parsing</a> and <a href=
-"http://en.wikipedia.org/wiki/Operator-precedence_parser">Operator-Precedence
-Parsing</a> 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.</p>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2><a name="ast">The Abstract Syntax Tree (AST)</a></h2>
-<!-- *********************************************************************** -->
-
-<div>
-
-<p>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:</p>
-
-<div class="doc_code">
-<pre>
-/// ExprAST - Base class for all expression nodes.
-class ExprAST {
-public:
- virtual ~ExprAST() {}
-};
-
-/// NumberExprAST - Expression class for numeric literals like "1.0".
-class NumberExprAST : public ExprAST {
- double Val;
-public:
- NumberExprAST(double val) : Val(val) {}
-};
-</pre>
-</div>
-
-<p>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 NumberExprAST class 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.</p>
-
-<p>Right now we only create the AST, so there are no useful accessor methods on
-them. It would be very easy to add a virtual method 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:
-</p>
-
-<div class="doc_code">
-<pre>
-/// VariableExprAST - Expression class for referencing a variable, like "a".
-class VariableExprAST : public ExprAST {
- std::string Name;
-public:
- VariableExprAST(const std::string &name) : Name(name) {}
-};
-
-/// BinaryExprAST - Expression class for a binary operator.
-class BinaryExprAST : public ExprAST {
- char Op;
- ExprAST *LHS, *RHS;
-public:
- BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs)
- : Op(op), LHS(lhs), RHS(rhs) {}
-};
-
-/// CallExprAST - Expression class for function calls.
-class CallExprAST : public ExprAST {
- std::string Callee;
- std::vector<ExprAST*> Args;
-public:
- CallExprAST(const std::string &callee, std::vector<ExprAST*> &args)
- : Callee(callee), Args(args) {}
-};
-</pre>
-</div>
-
-<p>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.</p>
-
-<p>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:</p>
-
-<div class="doc_code">
-<pre>
-/// PrototypeAST - This class represents the "prototype" for a function,
-/// which captures its name, and its argument names (thus implicitly the number
-/// of arguments the function takes).
-class PrototypeAST {
- std::string Name;
- std::vector<std::string> Args;
-public:
- PrototypeAST(const std::string &name, const std::vector<std::string> &args)
- : Name(name), Args(args) {}
-};
-
-/// FunctionAST - This class represents a function definition itself.
-class FunctionAST {
- PrototypeAST *Proto;
- ExprAST *Body;
-public:
- FunctionAST(PrototypeAST *proto, ExprAST *body)
- : Proto(proto), Body(body) {}
-};
-</pre>
-</div>
-
-<p>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 "ExprAST" class would probably have a type field.</p>
-
-<p>With this scaffolding, we can now talk about parsing expressions and function
-bodies in Kaleidoscope.</p>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2><a name="parserbasics">Parser Basics</a></h2>
-<!-- *********************************************************************** -->
-
-<div>
-
-<p>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:</p>
-
-<div class="doc_code">
-<pre>
- ExprAST *X = new VariableExprAST("x");
- ExprAST *Y = new VariableExprAST("y");
- ExprAST *Result = new BinaryExprAST('+', X, Y);
-</pre>
-</div>
-
-<p>In order to do this, we'll start by defining some basic helper routines:</p>
-
-<div class="doc_code">
-<pre>
-/// CurTok/getNextToken - Provide a simple token buffer. CurTok is the current
-/// token the parser is looking at. getNextToken reads another token from the
-/// lexer and updates CurTok with its results.
-static int CurTok;
-static int getNextToken() {
- return CurTok = gettok();
-}
-</pre>
-</div>
-
-<p>
-This implements a simple token buffer around the lexer. This allows
-us to look one token ahead at what the lexer is returning. Every function in
-our parser will assume that CurTok is the current token that needs to be
-parsed.</p>
-
-<div class="doc_code">
-<pre>
-
-/// Error* - These are little helper functions for error handling.
-ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
-PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
-FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
-</pre>
-</div>
-
-<p>
-The <tt>Error</tt> routines are simple helper routines that our parser will use
-to handle errors. 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
-routines make it easier to handle errors in routines that have various return
-types: they always return null.</p>
-
-<p>With these basic helper functions, we can implement the first
-piece of our grammar: numeric literals.</p>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2><a name="parserprimexprs">Basic Expression Parsing</a></h2>
-<!-- *********************************************************************** -->
-
-<div>
-
-<p>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. For numeric literals, we have:
-</p>
-
-<div class="doc_code">
-<pre>
-/// numberexpr ::= number
-static ExprAST *ParseNumberExpr() {
- ExprAST *Result = new NumberExprAST(NumVal);
- getNextToken(); // consume the number
- return Result;
-}
-</pre>
-</div>
-
-<p>This routine is very simple: it expects to be called when the current token
-is a <tt>tok_number</tt> token. It takes the current number value, creates
-a <tt>NumberExprAST</tt> node, advances the lexer to the next token, and finally
-returns.</p>
-
-<p>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:</p>
-
-<div class="doc_code">
-<pre>
-/// parenexpr ::= '(' expression ')'
-static ExprAST *ParseParenExpr() {
- getNextToken(); // eat (.
- ExprAST *V = ParseExpression();
- if (!V) return 0;
-
- if (CurTok != ')')
- return Error("expected ')'");
- getNextToken(); // eat ).
- return V;
-}
-</pre>
-</div>
-
-<p>This function illustrates a number of interesting things about the
-parser:</p>
-
-<p>
-1) It shows how we use the Error routines. 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
-return null on an error.</p>
-
-<p>2) Another interesting aspect of this function is that it uses recursion by
-calling <tt>ParseExpression</tt> (we will soon see that <tt>ParseExpression</tt> can call
-<tt>ParseParenExpr</tt>). 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.</p>
-
-<p>The next simple production is for handling variable references and function
-calls:</p>
-
-<div class="doc_code">
-<pre>
-/// identifierexpr
-/// ::= identifier
-/// ::= identifier '(' expression* ')'
-static ExprAST *ParseIdentifierExpr() {
- std::string IdName = IdentifierStr;
-
- getNextToken(); // eat identifier.
-
- if (CurTok != '(') // Simple variable ref.
- return new VariableExprAST(IdName);
-
- // Call.
- getNextToken(); // eat (
- std::vector<ExprAST*> Args;
- if (CurTok != ')') {
- while (1) {
- ExprAST *Arg = ParseExpression();
- if (!Arg) return 0;
- Args.push_back(Arg);
-
- if (CurTok == ')') break;
-
- if (CurTok != ',')
- return Error("Expected ')' or ',' in argument list");
- getNextToken();
- }
- }
-
- // Eat the ')'.
- getNextToken();
-
- return new CallExprAST(IdName, Args);
-}
-</pre>
-</div>
-
-<p>This routine follows the same style as the other routines. (It expects to be
-called if the current token is a <tt>tok_identifier</tt> token). It also has
-recursion and error handling. One interesting aspect of this is that it uses
-<em>look-ahead</em> 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 <tt>VariableExprAST</tt> or <tt>CallExprAST</tt> node as appropriate.
-</p>
-
-<p>Now that we have all of our simple expression-parsing logic in place, we can
-define a helper function to wrap it together into one entry point. We call this
-class of expressions "primary" expressions, for reasons that will become more
-clear <a href="LangImpl6.html#unary">later in the tutorial</a>. In order to
-parse an arbitrary primary expression, we need to determine what sort of
-expression it is:</p>
-
-<div class="doc_code">
-<pre>
-/// primary
-/// ::= identifierexpr
-/// ::= numberexpr
-/// ::= parenexpr
-static ExprAST *ParsePrimary() {
- switch (CurTok) {
- default: return Error("unknown token when expecting an expression");
- case tok_identifier: return ParseIdentifierExpr();
- case tok_number: return ParseNumberExpr();
- case '(': return ParseParenExpr();
- }
-}
-</pre>
-</div>
-
-<p>Now that you see the definition of this function, it is more obvious why we
-can assume the state of CurTok in the various functions. This uses look-ahead
-to determine which sort of expression is being inspected, and then parses it
-with a function call.</p>
-
-<p>Now that basic expressions are handled, we need to handle binary expressions.
-They are a bit more complex.</p>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2><a name="parserbinops">Binary Expression Parsing</a></h2>
-<!-- *********************************************************************** -->
-
-<div>
-
-<p>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 <em>precedence</em> than "+" (addition).</p>
-
-<p>There are many ways to handle this, but an elegant and efficient way is to
-use <a href=
-"http://en.wikipedia.org/wiki/Operator-precedence_parser">Operator-Precedence
-Parsing</a>. This parsing technique uses the precedence of binary operators to
-guide recursion. To start with, we need a table of precedences:</p>
-
-<div class="doc_code">
-<pre>
-/// BinopPrecedence - This holds the precedence for each binary operator that is
-/// defined.
-static std::map<char, int> BinopPrecedence;
-
-/// GetTokPrecedence - Get the precedence of the pending binary operator token.
-static int GetTokPrecedence() {
- if (!isascii(CurTok))
- return -1;
-
- // Make sure it's a declared binop.
- int TokPrec = BinopPrecedence[CurTok];
- if (TokPrec <= 0) return -1;
- return TokPrec;
-}
-
-int main() {
- // Install standard binary operators.
- // 1 is lowest precedence.
- BinopPrecedence['<'] = 10;
- BinopPrecedence['+'] = 20;
- BinopPrecedence['-'] = 20;
- BinopPrecedence['*'] = 40; // highest.
- ...
-}
-</pre>
-</div>
-
-<p>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
-<tt>GetTokPrecedence</tt> function returns the precedence for the current token,
-or -1 if the token is not a binary operator. Having a map 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 map
-and do the comparisons in the <tt>GetTokPrecedence</tt> function. (Or just use
-a fixed-size array).</p>
-
-<p>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.
-</p>
-
-<p>
-To start, an expression is a primary expression potentially followed by a
-sequence of [binop,primaryexpr] pairs:</p>
-
-<div class="doc_code">
-<pre>
-/// expression
-/// ::= primary binoprhs
-///
-static ExprAST *ParseExpression() {
- ExprAST *LHS = ParsePrimary();
- if (!LHS) return 0;
-
- return ParseBinOpRHS(0, LHS);
-}
-</pre>
-</div>
-
-<p><tt>ParseBinOpRHS</tt> 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
-<tt>ParseBinOpRHS</tt> and the current token is "+".</p>
-
-<p>The precedence value passed into <tt>ParseBinOpRHS</tt> indicates the <em>
-minimal operator precedence</em> that the function is allowed to eat. For
-example, if the current pair stream is [+, x] and <tt>ParseBinOpRHS</tt> is
-passed in a precedence of 40, it will not consume any tokens (because the
-precedence of '+' is only 20). With this in mind, <tt>ParseBinOpRHS</tt> starts
-with:</p>
-
-<div class="doc_code">
-<pre>
-/// binoprhs
-/// ::= ('+' primary)*
-static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
- // If this is a binop, find its precedence.
- while (1) {
- int TokPrec = GetTokPrecedence();
-
- // If this is a binop that binds at least as tightly as the current binop,
- // consume it, otherwise we are done.
- if (TokPrec < ExprPrec)
- return LHS;
-</pre>
-</div>
-
-<p>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:</p>
-
-<div class="doc_code">
-<pre>
- // Okay, we know this is a binop.
- int BinOp = CurTok;
- getNextToken(); // eat binop
-
- // Parse the primary expression after the binary operator.
- ExprAST *RHS = ParsePrimary();
- if (!RHS) return 0;
-</pre>
-</div>
-
-<p>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.</p>
-
-<p>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):</p>
-
-<div class="doc_code">
-<pre>
- // If BinOp binds less tightly with RHS than the operator after RHS, let
- // the pending operator take RHS as its LHS.
- int NextPrec = GetTokPrecedence();
- if (TokPrec < NextPrec) {
-</pre>
-</div>
-
-<p>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:</p>
-
-<div class="doc_code">
-<pre>
- ... if body omitted ...
- }
-
- // Merge LHS/RHS.
- LHS = new BinaryExprAST(BinOp, LHS, RHS);
- } // loop around to the top of the while loop.
-}
-</pre>
-</div>
-
-<p>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.</p>
-
-<p>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):</p>
-
-<div class="doc_code">
-<pre>
- // If BinOp binds less tightly with RHS than the operator after RHS, let
- // the pending operator take RHS as its LHS.
- int NextPrec = GetTokPrecedence();
- if (TokPrec < NextPrec) {
- <b>RHS = ParseBinOpRHS(TokPrec+1, RHS);
- if (RHS == 0) return 0;</b>
- }
- // Merge LHS/RHS.
- LHS = new BinaryExprAST(BinOp, LHS, RHS);
- } // loop around to the top of the while loop.
-}
-</pre>
-</div>
-
-<p>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 <tt>ParseBinOpRHS</tt> function specifying "TokPrec+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.</p>
-
-<p>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.
-</p>
-
-<p>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.</p>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2><a name="parsertop">Parsing the Rest</a></h2>
-<!-- *********************************************************************** -->
-
-<div>
-
-<p>
-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):
-</p>
-
-<div class="doc_code">
-<pre>
-/// prototype
-/// ::= id '(' id* ')'
-static PrototypeAST *ParsePrototype() {
- if (CurTok != tok_identifier)
- return ErrorP("Expected function name in prototype");
-
- std::string FnName = IdentifierStr;
- getNextToken();
-
- if (CurTok != '(')
- return ErrorP("Expected '(' in prototype");
-
- // Read the list of argument names.
- std::vector<std::string> ArgNames;
- while (getNextToken() == tok_identifier)
- ArgNames.push_back(IdentifierStr);
- if (CurTok != ')')
- return ErrorP("Expected ')' in prototype");
-
- // success.
- getNextToken(); // eat ')'.
-
- return new PrototypeAST(FnName, ArgNames);
-}
-</pre>
-</div>
-
-<p>Given this, a function definition is very simple, just a prototype plus
-an expression to implement the body:</p>
-
-<div class="doc_code">
-<pre>
-/// definition ::= 'def' prototype expression
-static FunctionAST *ParseDefinition() {
- getNextToken(); // eat def.
- PrototypeAST *Proto = ParsePrototype();
- if (Proto == 0) return 0;
-
- if (ExprAST *E = ParseExpression())
- return new FunctionAST(Proto, E);
- return 0;
-}
-</pre>
-</div>
-
-<p>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:</p>
-
-<div class="doc_code">
-<pre>
-/// external ::= 'extern' prototype
-static PrototypeAST *ParseExtern() {
- getNextToken(); // eat extern.
- return ParsePrototype();
-}
-</pre>
-</div>
-
-<p>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:</p>
-
-<div class="doc_code">
-<pre>
-/// toplevelexpr ::= expression
-static FunctionAST *ParseTopLevelExpr() {
- if (ExprAST *E = ParseExpression()) {
- // Make an anonymous proto.
- PrototypeAST *Proto = new PrototypeAST("", std::vector<std::string>());
- return new FunctionAST(Proto, E);
- }
- return 0;
-}
-</pre>
-</div>
-
-<p>Now that we have all the pieces, let's build a little driver that will let us
-actually <em>execute</em> this code we've built!</p>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2><a name="driver">The Driver</a></h2>
-<!-- *********************************************************************** -->
-
-<div>
-
-<p>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 <a href="#code">below</a> for full code in the "Top-Level
-Parsing" section.</p>
-
-<div class="doc_code">
-<pre>
-/// top ::= definition | external | expression | ';'
-static void MainLoop() {
- while (1) {
- fprintf(stderr, "ready> ");
- switch (CurTok) {
- case tok_eof: return;
- case ';': getNextToken(); break; // ignore top-level semicolons.
- case tok_def: HandleDefinition(); break;
- case tok_extern: HandleExtern(); break;
- default: HandleTopLevelExpression(); break;
- }
- }
-}
-</pre>
-</div>
-
-<p>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.</p>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2><a name="conclusions">Conclusions</a></h2>
-<!-- *********************************************************************** -->
-
-<div>
-
-<p>With just under 400 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:</p>
-
-<div class="doc_code">
-<pre>
-$ <b>./a.out</b>
-ready> <b>def foo(x y) x+foo(y, 4.0);</b>
-Parsed a function definition.
-ready> <b>def foo(x y) x+y y;</b>
-Parsed a function definition.
-Parsed a top-level expr
-ready> <b>def foo(x y) x+y );</b>
-Parsed a function definition.
-Error: unknown token when expecting an expression
-ready> <b>extern sin(a);</b>
-ready> Parsed an extern
-ready> <b>^D</b>
-$
-</pre>
-</div>
-
-<p>There is a lot of room for extension here. You can define new AST nodes,
-extend the language in many ways, etc. In the <a href="LangImpl3.html">next
-installment</a>, we will describe how to generate LLVM Intermediate
-Representation (IR) from the AST.</p>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2><a name="code">Full Code Listing</a></h2>
-<!-- *********************************************************************** -->
-
-<div>
-
-<p>
-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 C and C++ standard libraries, of
-course.) To build this, just compile with:</p>
-
-<div class="doc_code">
-<pre>
-# Compile
-clang++ -g -O3 toy.cpp
-# Run
-./a.out
-</pre>
-</div>
-
-<p>Here is the code:</p>
-
-<div class="doc_code">
-<pre>
-#include <cstdio>
-#include <cstdlib>
-#include <string>
-#include <map>
-#include <vector>
-
-//===----------------------------------------------------------------------===//
-// Lexer
-//===----------------------------------------------------------------------===//
-
-// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
-// of these for known things.
-enum Token {
- tok_eof = -1,
-
- // commands
- tok_def = -2, tok_extern = -3,
-
- // primary
- tok_identifier = -4, tok_number = -5
-};
-
-static std::string IdentifierStr; // Filled in if tok_identifier
-static double NumVal; // Filled in if tok_number
-
-/// gettok - Return the next token from standard input.
-static int gettok() {
- static int LastChar = ' ';
-
- // Skip any whitespace.
- while (isspace(LastChar))
- LastChar = getchar();
-
- if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
- IdentifierStr = LastChar;
- while (isalnum((LastChar = getchar())))
- IdentifierStr += LastChar;
-
- if (IdentifierStr == "def") return tok_def;
- if (IdentifierStr == "extern") return tok_extern;
- return tok_identifier;
- }
-
- if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+
- std::string NumStr;
- do {
- NumStr += LastChar;
- LastChar = getchar();
- } while (isdigit(LastChar) || LastChar == '.');
-
- NumVal = strtod(NumStr.c_str(), 0);
- return tok_number;
- }
-
- if (LastChar == '#') {
- // Comment until end of line.
- do LastChar = getchar();
- while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
-
- if (LastChar != EOF)
- return gettok();
- }
-
- // Check for end of file. Don't eat the EOF.
- if (LastChar == EOF)
- return tok_eof;
-
- // Otherwise, just return the character as its ascii value.
- int ThisChar = LastChar;
- LastChar = getchar();
- return ThisChar;
-}
-
-//===----------------------------------------------------------------------===//
-// Abstract Syntax Tree (aka Parse Tree)
-//===----------------------------------------------------------------------===//
-
-/// ExprAST - Base class for all expression nodes.
-class ExprAST {
-public:
- virtual ~ExprAST() {}
-};
-
-/// NumberExprAST - Expression class for numeric literals like "1.0".
-class NumberExprAST : public ExprAST {
- double Val;
-public:
- NumberExprAST(double val) : Val(val) {}
-};
-
-/// VariableExprAST - Expression class for referencing a variable, like "a".
-class VariableExprAST : public ExprAST {
- std::string Name;
-public:
- VariableExprAST(const std::string &name) : Name(name) {}
-};
-
-/// BinaryExprAST - Expression class for a binary operator.
-class BinaryExprAST : public ExprAST {
- char Op;
- ExprAST *LHS, *RHS;
-public:
- BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs)
- : Op(op), LHS(lhs), RHS(rhs) {}
-};
-
-/// CallExprAST - Expression class for function calls.
-class CallExprAST : public ExprAST {
- std::string Callee;
- std::vector<ExprAST*> Args;
-public:
- CallExprAST(const std::string &callee, std::vector<ExprAST*> &args)
- : Callee(callee), Args(args) {}
-};
-
-/// PrototypeAST - This class represents the "prototype" for a function,
-/// which captures its name, and its argument names (thus implicitly the number
-/// of arguments the function takes).
-class PrototypeAST {
- std::string Name;
- std::vector<std::string> Args;
-public:
- PrototypeAST(const std::string &name, const std::vector<std::string> &args)
- : Name(name), Args(args) {}
-
-};
-
-/// FunctionAST - This class represents a function definition itself.
-class FunctionAST {
- PrototypeAST *Proto;
- ExprAST *Body;
-public:
- FunctionAST(PrototypeAST *proto, ExprAST *body)
- : Proto(proto), Body(body) {}
-
-};
-
-//===----------------------------------------------------------------------===//
-// Parser
-//===----------------------------------------------------------------------===//
-
-/// CurTok/getNextToken - Provide a simple token buffer. CurTok is the current
-/// token the parser is looking at. getNextToken reads another token from the
-/// lexer and updates CurTok with its results.
-static int CurTok;
-static int getNextToken() {
- return CurTok = gettok();
-}
-
-/// BinopPrecedence - This holds the precedence for each binary operator that is
-/// defined.
-static std::map<char, int> BinopPrecedence;
-
-/// GetTokPrecedence - Get the precedence of the pending binary operator token.
-static int GetTokPrecedence() {
- if (!isascii(CurTok))
- return -1;
-
- // Make sure it's a declared binop.
- int TokPrec = BinopPrecedence[CurTok];
- if (TokPrec <= 0) return -1;
- return TokPrec;
-}
-
-/// Error* - These are little helper functions for error handling.
-ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
-PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
-FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
-
-static ExprAST *ParseExpression();
-
-/// identifierexpr
-/// ::= identifier
-/// ::= identifier '(' expression* ')'
-static ExprAST *ParseIdentifierExpr() {
- std::string IdName = IdentifierStr;
-
- getNextToken(); // eat identifier.
-
- if (CurTok != '(') // Simple variable ref.
- return new VariableExprAST(IdName);
-
- // Call.
- getNextToken(); // eat (
- std::vector<ExprAST*> Args;
- if (CurTok != ')') {
- while (1) {
- ExprAST *Arg = ParseExpression();
- if (!Arg) return 0;
- Args.push_back(Arg);
-
- if (CurTok == ')') break;
-
- if (CurTok != ',')
- return Error("Expected ')' or ',' in argument list");
- getNextToken();
- }
- }
-
- // Eat the ')'.
- getNextToken();
-
- return new CallExprAST(IdName, Args);
-}
-
-/// numberexpr ::= number
-static ExprAST *ParseNumberExpr() {
- ExprAST *Result = new NumberExprAST(NumVal);
- getNextToken(); // consume the number
- return Result;
-}
-
-/// parenexpr ::= '(' expression ')'
-static ExprAST *ParseParenExpr() {
- getNextToken(); // eat (.
- ExprAST *V = ParseExpression();
- if (!V) return 0;
-
- if (CurTok != ')')
- return Error("expected ')'");
- getNextToken(); // eat ).
- return V;
-}
-
-/// primary
-/// ::= identifierexpr
-/// ::= numberexpr
-/// ::= parenexpr
-static ExprAST *ParsePrimary() {
- switch (CurTok) {
- default: return Error("unknown token when expecting an expression");
- case tok_identifier: return ParseIdentifierExpr();
- case tok_number: return ParseNumberExpr();
- case '(': return ParseParenExpr();
- }
-}
-
-/// binoprhs
-/// ::= ('+' primary)*
-static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
- // If this is a binop, find its precedence.
- while (1) {
- int TokPrec = GetTokPrecedence();
-
- // If this is a binop that binds at least as tightly as the current binop,
- // consume it, otherwise we are done.
- if (TokPrec < ExprPrec)
- return LHS;
-
- // Okay, we know this is a binop.
- int BinOp = CurTok;
- getNextToken(); // eat binop
-
- // Parse the primary expression after the binary operator.
- ExprAST *RHS = ParsePrimary();
- if (!RHS) return 0;
-
- // If BinOp binds less tightly with RHS than the operator after RHS, let
- // the pending operator take RHS as its LHS.
- int NextPrec = GetTokPrecedence();
- if (TokPrec < NextPrec) {
- RHS = ParseBinOpRHS(TokPrec+1, RHS);
- if (RHS == 0) return 0;
- }
-
- // Merge LHS/RHS.
- LHS = new BinaryExprAST(BinOp, LHS, RHS);
- }
-}
-
-/// expression
-/// ::= primary binoprhs
-///
-static ExprAST *ParseExpression() {
- ExprAST *LHS = ParsePrimary();
- if (!LHS) return 0;
-
- return ParseBinOpRHS(0, LHS);
-}
-
-/// prototype
-/// ::= id '(' id* ')'
-static PrototypeAST *ParsePrototype() {
- if (CurTok != tok_identifier)
- return ErrorP("Expected function name in prototype");
-
- std::string FnName = IdentifierStr;
- getNextToken();
-
- if (CurTok != '(')
- return ErrorP("Expected '(' in prototype");
-
- std::vector<std::string> ArgNames;
- while (getNextToken() == tok_identifier)
- ArgNames.push_back(IdentifierStr);
- if (CurTok != ')')
- return ErrorP("Expected ')' in prototype");
-
- // success.
- getNextToken(); // eat ')'.
-
- return new PrototypeAST(FnName, ArgNames);
-}
-
-/// definition ::= 'def' prototype expression
-static FunctionAST *ParseDefinition() {
- getNextToken(); // eat def.
- PrototypeAST *Proto = ParsePrototype();
- if (Proto == 0) return 0;
-
- if (ExprAST *E = ParseExpression())
- return new FunctionAST(Proto, E);
- return 0;
-}
-
-/// toplevelexpr ::= expression
-static FunctionAST *ParseTopLevelExpr() {
- if (ExprAST *E = ParseExpression()) {
- // Make an anonymous proto.
- PrototypeAST *Proto = new PrototypeAST("", std::vector<std::string>());
- return new FunctionAST(Proto, E);
- }
- return 0;
-}
-
-/// external ::= 'extern' prototype
-static PrototypeAST *ParseExtern() {
- getNextToken(); // eat extern.
- return ParsePrototype();
-}
-
-//===----------------------------------------------------------------------===//
-// Top-Level parsing
-//===----------------------------------------------------------------------===//
-
-static void HandleDefinition() {
- if (ParseDefinition()) {
- fprintf(stderr, "Parsed a function definition.\n");
- } else {
- // Skip token for error recovery.
- getNextToken();
- }
-}
-
-static void HandleExtern() {
- if (ParseExtern()) {
- fprintf(stderr, "Parsed an extern\n");
- } else {
- // Skip token for error recovery.
- getNextToken();
- }
-}
-
-static void HandleTopLevelExpression() {
- // Evaluate a top-level expression into an anonymous function.
- if (ParseTopLevelExpr()) {
- fprintf(stderr, "Parsed a top-level expr\n");
- } else {
- // Skip token for error recovery.
- getNextToken();
- }
-}
-
-/// top ::= definition | external | expression | ';'
-static void MainLoop() {
- while (1) {
- fprintf(stderr, "ready> ");
- switch (CurTok) {
- case tok_eof: return;
- case ';': getNextToken(); break; // ignore top-level semicolons.
- case tok_def: HandleDefinition(); break;
- case tok_extern: HandleExtern(); break;
- default: HandleTopLevelExpression(); break;
- }
- }
-}
-
-//===----------------------------------------------------------------------===//
-// Main driver code.
-//===----------------------------------------------------------------------===//
-
-int main() {
- // Install standard binary operators.
- // 1 is lowest precedence.
- BinopPrecedence['<'] = 10;
- BinopPrecedence['+'] = 20;
- BinopPrecedence['-'] = 20;
- BinopPrecedence['*'] = 40; // highest.
-
- // Prime the first token.
- fprintf(stderr, "ready> ");
- getNextToken();
-
- // Run the main "interpreter loop" now.
- MainLoop();
-
- return 0;
-}
-</pre>
-</div>
-<a href="LangImpl3.html">Next: Implementing Code Generation to LLVM IR</a>
-</div>
-
-<!-- *********************************************************************** -->
-<hr>
-<address>
- <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
- src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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-
- <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
- <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
- Last modified: $Date$
-</address>
-</body>
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