Compile with function definition inside

Question

How can I compile a code with target C containing function definition inside?

I saw post using With. But this just inserts the function in the appropriate place like macros in C. Suggestion with With

    f = Compile[{{x, _Real}},
      g[y] = y^2;

      g[x]
      , CompilationTarget -> "C"
      ]

Calls Main:

In[12]:= CompilePrint[f]


1 argument
        3 Real registers
        Underflow checking off
        Overflow checking off
        Integer overflow checking on
        RuntimeAttributes -> {}

        R0 = A1
        Result = R2

1   V17 = MainEvaluate[                       2
Function[{x}, g[y] = y ][ R0]]
2   R2 = MainEvaluate[ Hold[g][ R0]]
3   Return

Answer 1

First define the function normally:

g[y_] := y^2;

Then substitute the definition in:

f = ReleaseHold[Hold[Compile[{{x, _Real}},
 g[x], CompilationTarget -> "C"]] /. DownValues[g]]

And there is no longer MainEvaluatein the compiled function.

As Oleksandr R. suggested, you can also do

g = #^2 &;
f = Compile[{{x, _Real}},
  g[x], CompilationTarget -> "C", 
  CompilationOptions -> {"InlineExternalDefinitions" -> True}]

Answer 2

Your question probably will not get the answer you are looking for, because although it is technically possible, getting Compile to work properly is hard enough already without the added difficulty of automatically preprocessing its input. Thus I think that this approach will tend to obscure and complicate the job, rather than making anything easier.

The simple answer is: it's not possible because the VM doesn't have any concept of a "function type". The only allowable types are numeric quantities or void. So, you can write

Compile[{{x, _Real}}, Function[{y}, y^2][x]]

and it compiles and operates just as it should. But you can't have

Compile[{{x, _Real}}, Module[{g}, g = Function[{y}, y^2]; g[x]]]

because g is not something that can be represented inside the VM. The code of the function can be, but the treatment of that code as a named quantity is an abstraction too far.