Labs 02: Lexer for Tool
Caution: We are no longer going to use the parser+trees given in
lib/toolc-frontend_2.11-3.0.jar. This commit removes this file.
To merge the stubs for the Lexer into your current branch (which should contain a working Evaluator by now), use
git merge origin/Lab02
The structure of your project directory should be as follows:
toolc ├── Main.scala (updated) │ ├── ast │ └── Trees.scala (new, for later) │ ├── eval │ └── Evaluator.scala │ ├── lexer │ ├── Lexer.scala (new) │ └── Tokens.scala (new) │ └── utils ├── Context.scala (new) ├── Positioned.scala (new) ├── Reporter.scala (new) └── Pipeline.scala (new)
This lab will focus on the following two files:
lexer/Tokens.scala: list of token kinds and tokens.
lexer/Lexer.scala: stub for the
A Lexer for Tool
The role of a lexer is to read the input text and convert it to a list of tokens. Tokens are the smallest useful units in a source file: a name referring to a variable, a bracket, a keyword etc. The role of the lexer is to group together those useful units (e.g. return the keyword
while as a unit, as opposed to individual characters
e) and to abstract away all useless information (i.e. whitespace, comments).
Here are some details you should pay attention to:
- Make sure you recognize keywords as their own token type.
while, for instance, should be lexed as a token
WHILE(), not as an identifier representing an object called “while”.
- Make sure you correctly register the position of all tokens. Note the
- In general, it is good to output as many errors as possible (this helps whoever uses your compiler). This means your lexer should not give up after the first error, but rather skip the erroneous token, emit an error message, and then continue lexing. Use the
BADtoken to mark erroneous tokens.
- The Lexer does not immediately read and return all tokens, it returns an Iterator[Token] that will be used by future phases to read tokens on demand.
- Comments should not produce tokens.
- Returned tokens should be fresh instances of the the appropriate Token subclass. Value tokens (tokens that carry a value, such as identifiers), need to be constructed with the appropriate value.
- The position of a token should be the position of its first character.
- You should apply the longest matching rule:
- Take special care at what is considered a valid identifier, integer literal, and string literal.
Your compiler needs to exit with a non-0 code in case it encounters an error. Bad tokens of invalid characters are considered as such errors.
Reporter available in the
ctx parameter of the
run method, provides a method
error which will store in the Reporter the information that an error has occurred.
Reporter also provides the method
terminateIfErrors which will terminate the compiler if an error has occurred before. This method is already called between each phase of the compiler. To make sure that the compiler also terminates with non-0 code in case the *last* phase emits an error, it is also called in the end of
(Notice that, since your lexer phase returns an Iterator, errors might be emitted by your lexer while consuming the iterator during the DisplayToken phase)
In short, make sure you call
error whenever you encounter an erroneous character, to make sure that your compiler will eventually exit with an error code. For the lexing phase, avoid
fatal as this will terminate the compiler immediately (and not allow you to report multiple errors).
You can find the lexer in the
Lexer.scala file. Its code is imperative and based on iterators, both for the input file and the output tokens.
The Lexer has the following components:
- The public method is
run. It returns an iterator of Tokens by repeatedly calling
EOF()token is found.
nextToken()is essentially a wrapper method around
readToken(). It removes whitespace and comments and then calls
readToken(). This method can be recursive if you want, but in this case make sure that it is tail-recursive so it does not stack overflow on long inputs (this is imposed with
readToken()is the main function of the lexer. It will look at the current character and decide what kind of token it has to emit, possibly reading more characters along the way.
- The input file is read by a class called
SourceReader. To make it easier for you,
SourceReaderwill cache two characters at a time so you can look ahead if needed. You have access to the two characters with
consumewill read a new character from the input file (there is no way to go back to previous characters, so structure your code carefully). Finally,
currentPosreturns the position of
currentChar. You can use the methods of the reader without qualification due to
- Finally, the
keywordsmethod should return a keyword token corresponding to a string, or
Noneif there is not corresponding keyword.
It is your task to complete
java -jar toolc-reference-3.0.jar --tokens <file.tool>
to show the tokens of any Tool source file.
PROGRAM()(1:1) ID(Factorial)(1:9) LBRACE()(1:19) PRINTLN()(2:5) LPAREN()(2:12) STRINGLIT(10! = )(2:13) PLUS()(2:22) NEW()(2:24) ID(Fact)(2:28) LPAREN()(2:32) RPAREN()(2:33) DOT()(2:34) ID(computeFactorial)(2:35) LPAREN()(2:51) INTLIT(10)(2:52) RPAREN()(2:54) RPAREN()(2:55) SEMICOLON()(2:56) RBRACE()(3:1) CLASS()(5:1) ID(Fact)(5:7) LBRACE()(5:12) DEF()(6:5) ID(computeFactorial)(6:9) LPAREN()(6:25) ID(num)(6:26) COLON()(6:30) INT()(6:32) RPAREN()(6:35) COLON()(6:37) INT()(6:39) EQSIGN()(6:43) LBRACE()(6:45) VAR()(7:9) ID(num_aux)(7:13) COLON()(7:21) INT()(7:23) SEMICOLON()(7:26) IF()(8:9) LPAREN()(8:12) ID(num)(8:13) LESSTHAN()(8:17) INTLIT(1)(8:19) RPAREN()(8:20) ID(num_aux)(9:13) EQSIGN()(9:21) INTLIT(1)(9:23) SEMICOLON()(9:24) ELSE()(10:9) ID(num_aux)(11:13) EQSIGN()(11:21) ID(num)(11:23) TIMES()(11:27) LPAREN()(11:29) THIS()(11:30) DOT()(11:34) ID(computeFactorial)(11:35) LPAREN()(11:51) ID(num)(11:52) MINUS()(11:56) INTLIT(1)(11:58) RPAREN()(11:59) RPAREN()(11:60) SEMICOLON()(11:61) RETURN()(12:9) ID(num_aux)(12:16) SEMICOLON()(12:23) RBRACE()(13:5) RBRACE()(14:1) EOF()(14:2)
Note that the lexer emits
EOF and only then will the
Iterator.hasNext method return false.
You are given 2 weeks for this assignment.
Deadline: Tuesday, Oct. 11, 23.59pm.
You must use the interface on http://larasrv09.epfl.ch:9000/clp16/repository as follows:
- Click on the “Deliverables” tab
- Click “Deliver this version” on the commit corresponding to the version of your code you want to submit