error when defining function inside another function in fortran - function

Question

For some use I need to define a function inside another function inside a fortran module.
A sample code for easy comprehension is
module func
implicit none
contains
real function f(x,y)
real x,y,g
real function g(r)
real r
g=r
end function g
f=x*g(y)
end function f
end module func
use func
implicit none
write(*,*) f(1.0,1.0)
end
This is giving lots of errors in gfortran like unexpected data declaration, expected end function f, not g....etc.
What is the correct way of defining a function inside another function in fortran?

Answer 1

You use an internal subprogram, see below. Note internal subprograms themselves can not contain internal subprograms.
ian#eris:~/work/stack$ cat contained.f90
Module func
Implicit None
Contains
Real Function f(x,y)
! Interface explicit so don't need to declare g
Real x,y
f=x*g(y)
Contains
Real Function g(r)
Real r
g=r
End Function g
End Function f
End Module func
Program testit
Use func
Implicit None
Write(*,*) f(1.0,1.0)
End Program testit
ian#eris:~/work/stack$ gfortran-8 -std=f2008 -Wall -Wextra -fcheck=all -O -g contained.f90
ian#eris:~/work/stack$ ./a.out
1.00000000
ian#eris:~/work/stack$