FunctionDeclaration can be used in FunctionCompile:

The compilation uses the function declaration:

FunctionDeclaration can be used to add a definition to the default compiler environment:

Use the declaration in a compilation:

The compilation uses the function declaration:

Reset the compiler environment to clear the declaration:

FunctionDeclaration can be used to add a definition to a new compiler environment:

Use the CompilerEnvironment option to specify the environment:

A function name can be used in more than one declaration:

Call one version:

Call the other version:

Reset the compiler environment to clear the declaration:

A function declaration can call another declaration:

Compilation uses both declarations:

A function declaration can work for more than one type:

The declaration works for an integer argument:

The same declaration works for a string:

A function declaration can use definitions created with := to set up DownValues:

Give the function a name and a type and state that the implementation comes from the DownValues:

Now these definitions are converted into efficient compiled code:

What is neat is that this workflow is compatible with the uncompiled version:

The optimized compiled code is much faster:

A function declaration can be made to call the Wolfram Engine for a computation:

Call FactorInteger but give the function a different name in compiled code:

Now a function that uses this declaration is compiled:

The computation is done in the Wolfram Engine, so the compiled code may not have an advantage:

TypeOf and TypeEvaluate can be used to set up the types in a declaration: