alex

Analyze text from stdin

$ echo [His network looks good] | alex --stdin

$ alex

$ alex [path/to/file.md]

$ alex *.md ![example.md]

alex [options] <input_file.x>

-o, --output=FILE
    Specifies the name of the output Haskell file to be generated. By default, Alex writes to a file with the same base name as the input but with a .hs extension.

-g, --ghc
    Generates code that is specifically optimized for use with the Glasgow Haskell Compiler (GHC), potentially offering better performance or compatibility.

-s, --strict
    Generates a 'strict' lexer. This can influence how the lexer handles lazy evaluation in Haskell, often leading to more predictable performance and memory usage.

-v, --version
    Displays the version information of the Alex tool and then exits.

-h, --help
    Prints a comprehensive help message detailing all available command-line options and their usage, then exits.

--encoding=ENC
    Specifies the character encoding of the input Alex file (e.g., UTF-8). This ensures correct processing of multi-byte characters in regular expressions and actions.

--monad
    Generates a monadic lexer, allowing the lexer actions to operate within a monadic context (e.g., IO, State). This is useful for lexers that require side effects or state management.

--bytestring
    Generates a lexer that operates on ByteString input rather than String. This is often preferred for performance and memory efficiency when dealing with large binary or text data.

--utf8
    Configures the generated lexer to handle UTF-8 encoded input correctly, ensuring that multi-byte Unicode characters are recognized and processed as single characters.

The command alex refers to Alex, a powerful lexical analyzer generator specifically designed for the Haskell programming language. It serves a fundamental role in the initial phase of compilation, commonly known as lexing or scanning. Alex takes a high-level description of tokens defined using regular expressions, along with corresponding Haskell actions, specified in an .x source file. From this input, it efficiently generates a Haskell module (typically an .hs file) that can read an input character stream and produce a sequence of recognized tokens.

Lexical analysis is the process of transforming a raw sequence of characters into a stream of tokens—meaningful units in a programming language, such as keywords, identifiers, operators, or literals. Alex automates this often complex and error-prone task, performing a function analogous to flex and lex in the C/C++ programming ecosystem. While not a general-purpose Linux utility in the sense of commands like ls or grep, Alex is an indispensable tool for Haskell developers engaged in building compilers, interpreters, or any application that necessitates robust text parsing and understanding of language constructs. It leverages finite automata theory to produce highly optimized and performant lexers.

It is important to note that alex is a specialized development tool for the Haskell programming language, not a general-purpose utility found on all Linux systems by default. Its presence and functionality are contingent upon a Haskell development environment being installed. The Haskell code generated by Alex then needs to be compiled using a Haskell compiler like ghc. Users attempting to run alex without understanding its specific context in compiler construction and Haskell development may encounter unexpected results or errors.

In a typical compilation pipeline, the output generated by alex (the lexical analyzer or 'lexer') serves as the input for a parser. The parser then takes this stream of tokens and constructs a syntax tree, verifying the grammatical correctness of the input program. This clear separation of concerns—lexing for tokenizing, and parsing for structuring—significantly simplifies the overall language processing task.

Alex is conceptually the Haskell equivalent of flex (Fast Lexical Analyzer Generator), a widely used tool in the C/C++ world. Both tools generate efficient lexical analyzers from specifications based on regular expressions. The primary distinction lies in the target programming language for the generated code: Alex produces Haskell code, while flex generates C or C++.

Alex was developed by Chris Dornan and has been maintained as an integral part of the broader Haskell ecosystem. It provides a robust, modern, and efficient alternative to traditional lexical analyzer generators for Haskell, integrating seamlessly with the language's features and strong type system. Its ongoing development is driven by the needs of the Haskell community for powerful compiler-building tools, ensuring it evolves to support new language features and performance improvements.

flex(1), lex(1), happy(1), ghc(1)