3/20/2024 0 Comments Lex and yacc online compilerThe right-hand-side of these let statements is not normal OCAML code, but instead a regular expression following the description in section 12.2.4 of the manual. Both this method and the type declaration could have gone into a separate ml or mli file instead, and then loaded in.įollowing the preamble is a series of let statements for providing shortcuts of regular expressions. I have also included here a method that produces a string representation of the token, for printing purposes. For us these are opening the Lexing module so we have easy access to its methods (i.e. we could write foo instead of Lexing.foo some times), and specifying the type of the tokens we want to produce (we can call the type anything we want of course). We can set up some initial instructions here. In this case there is a first section of the file enclosed in braces, called the preamble. These files typically look like OCAML files, but with some specific and different syntax rules. ![]() | FLOAT f -> "FLOAT " ^ string_of_float f ![]() Here is how a sample file might look like for ocamllex (this file resides in ocaml/parsing/lexer.mll): open Lexing Lexical Analysis and lexersĪ lexer expects as input a file that is in a very specific form, which differs from language to language, but in general shares some features. We will take a glimpse at the syntactic analysis/parsing portion when we cover context-free grammars. This step will depend on the specific processor architecture used and what processor instructions are available.įor the time being, we will only concern ourselves with the lexical analysis portion. In modern compilers, this step constitutes the bulk of the compiler’s work.Ĭode Generation At this stage the intermediate language code is turned into machine instructions and is written into an executable file, or typically assembly code. This represention is agnostic to the computer architecture, but is close enough that generating machine code from it is relatively easy.Ĭode Optimization A number of techniques exist to try to optimize the intermediate code, storing often-used computations, removing steps that can be avoided, inlining some function calls and others. Intermediate Code Generation Typically a next step is to turn the AST into some intermediate language code, with a small set of instructions. This could include for example type-checking. Semantic Analysis During semantic analysis we analyze the AST produced by the syntactic analysis step, to determine the “meaning” of the individual parts. Tools called “parser generators” produce a module that performs this step based on descriptions for context-free-languages that we will see in the future. ![]() This is effectively the step in normal language that takes the words and forms them into sentences. Syntactic Analysis During syntactic analysis we take this stream of lexemes produced by the lexical analysis phase, and turn these into a more abstract form, typically called the abstract syntax tree (AST). The module produced by the lexer can then be included in the other steps. ![]() Users typically describe these lexemes via regular expressions, and the Lexer is a program that takes this description and produces a DFA that drives this separation of the text in lexemes. The theory of finite automata and regular expression is the essential ingredient of standard lexical analysis tools, typically called Lexers.īriefly a program when processed by a compiler goes through the following phases: Lexical Analysis During lexical analysis we take the input text file, and identify the lexemes, or tokens, i.e. the individual “words” that form the program.
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