Compilers: Principles, Techniques, and ToolsAho, Sethi, Ullman, Compilers: Principles, Techniques, and Tools, Addison-Wesley, 1986. is a computer science textbook by Alfred V.

<programming, tool> A program that converts another program from some source language (or programming language) to machine language (object code). Some compilers output assembly language which is then converted to {machine language} by a separate assembler. A compiler is distinguished from an assembler by the fact that each input statement does not, in general, correspond to a single machine instruction or fixed sequence of instructions. A compiler may support such features as automatic allocation of variables, arbitrary arithmetic expressions, control structures such as FOR and WHILE loops, variable scope, input/ouput operations, higher-order functions and portability of source code. AUTOCODER, written in 1952, was possibly the first primitive compiler. Laning and Zierler's compiler, written in 1953-1954, was possibly the first true working algebraic compiler. See also byte-code compiler, native compiler, {optimising compiler}. (1994-11-07)

In computing, a compiler is a computer program that translates computer code written in one programming language (the source language) into another language (the target language). The name "compiler" is primarily used for programs that translate source code from a high-level programming language to a lower level language (e.g. assembly language, object code, or machine code) to create an executable program.^: p1

There are many different types of compilers which produce output in different useful forms. A cross-compiler produces code for a different CPU or operating system than the one on which the cross-compiler itself runs. A bootstrap compiler is often a temporary compiler, used for compiling a more permanent or better optimised compiler for a language.

Related software include, a program that translates from a low-level language to a higher level one is a decompiler ; a program that translates between high-level languages, usually called a source-to-source compiler or transpiler. A language rewriter is usually a program that translates the form of expressions without a change of language. A compiler-compiler is a compiler that produces a compiler (or part of one), often in a generic and reusable way so as to be able to produce many differing compilers.

A compiler is likely to perform some or all of the following operations, often called phases: preprocessing, lexical analysis, parsing, semantic analysis (syntax-directed translation), conversion of input programs to an intermediate representation, code optimization and code generation. Compilers generally implement these phases as modular components, promoting efficient design and correctness of transformations of source input to target output. Program faults caused by incorrect compiler behavior can be very difficult to track down and work around; therefore, compiler implementers invest significant effort to ensure compiler correctness.

Compilers are not the only language processor used to transform source programs. An interpreter is computer software that transforms and then executes the indicated operations.^: p2 The translation process influences the design of computer languages, which leads to a preference of compilation or interpretation. In theory, a programming language can have both a compiler and an interpreter. In practice, programming languages tend to be associated with just one (a compiler or an interpreter).