D Programming Language 2.0

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Last update Thu Oct 1 22:59:52 2009

# Mixins

Mixins (not to be confused with template mixins) enable string constants to be compiled as regular D code and inserted into the program. Combining this with compile time manipulation of strings enables the creation of domain-specific languages.

For example, here we can create a template that generates a struct with the named members:

    template GenStruct(char[] Name, char[] M1)
    {
        const char[] GenStruct = "struct " ~ Name ~ "{ int " ~ M1 ~ "; }";
    }

    mixin(GenStruct!("Foo", "bar"));

which generates:

    struct Foo { int bar; }

Superficially, since D mixins can manipulate text and compile the result, it has some similar properties to the C preprocessor. But there are major, fundamental differences:

- The C preprocessing step occurs before lexical analysis. This makes it impossible to lex or parse C without access to all of the context, including all #include'd files, paths and all relevant compiler switches. Mixins occur during semantic analysis, and do not affect the lexing or parsing process. Lexing and parsing can still occur without semantic analysis.
- The C preprocessor can be used to create what appears to be different syntax:

      #define BEGIN {
      #define END   }

      BEGIN
        int x = 3;
        foo(x);
      END

  This monkey business is impossible with mixins. Mixed in text must form complete declarations, statements, or expressions.
- C macros will affect everything following that has the same name, even if they are in nested scopes. C macros cut across all scopes. This problem is called being not "coding hygenic". Mixins follow the usual scoping rules, and is hygenic.
- C preprocessing expressions follow a different syntax and have different semantic rules than the C language. The C preprocessor is technically a different language. Mixins are in the same language.
- C const declarations and C++ templates are invisible to C preprocessing. Mixins can be manipulated using templates and const declarations.

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