D Programming Language
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# Introduction

The D programming language is a general purpose systems programming language. To that end, a D program is a collection of modules that can be compiled separately to native code that is combined with libraries and compiled C code by a linker to create a native executable.

### Phases of Compilation

The process of compiling is divided into multiple phases. Each phase has no dependence on subsequent phases. For example, the scanner is not perturbed by the semantic analyzer. This separation of the passes makes language tools like syntax directed editors relatively easy to produce. It also is possible to compress D source by storing it in ‘tokenized’ form.

1. source character set
   The source file is checked to see what character set it is, and the appropriate scanner is loaded. ASCII and UTF formats are accepted.
2. script line
   If the first line starts with "#!", then that line is ignored.
3. lexical analysis
   The source file is divided up into a sequence of tokens. Special tokens are replaced with other tokens. SpecialTokenSequences are processed and removed.
4. syntax analysis
   The sequence of tokens is parsed to form syntax trees.
5. semantic analysis
   The syntax trees are traversed to declare variables, load symbol tables, assign types, and in general determine the meaning of the program.
6. optimization
   Optimization is an optional pass that tries to rewrite the program in a semantically equivalent, but faster executing, version.
7. code generation
   Instructions are selected from the target architecture to implement the semantics of the program. The typical result will be an object file, suitable for input to a linker.

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