Preliminary implementation of default values

src/CMakeLists.txt

@@ -2,6 +2,7 @@ set(SRC
     stdlib_experimental_ascii.f90
     stdlib_experimental_io.f90
     stdlib_experimental_error.f90
+    stdlib_experimental_default.f90
 )
 
 add_library(fortran_stdlib ${SRC})

src/Makefile.manual

@@ -1,6 +1,7 @@
 OBJS = stdlib_experimental_ascii.o \
        stdlib_experimental_error.o \
        stdlib_experimental_io.o \
+       stdlib_experimental_default.o \
 
 .PHONY: all clean
 .SUFFIXES: .f90 .o
@@ -15,6 +16,7 @@ all: $(OBJS)
 stdlib_experimental_ascii.o: stdlib_experimental_ascii.f90
 stdlib_experimental_error.o: stdlib_experimental_error.f90
 stdlib_experimental_io.o: stdlib_experimental_io.f90
+stdlib_experimental_default.o: stdlib_experimental_default.f90
 
 clean:
 	$(RM) *.o *.mod

src/stdlib_experimental_default.f90

@@ -0,0 +1,142 @@
+module default_m
+  !!
+  !! Provides a generic function `default`, which can be used to
+  !! conveniently implement fallback values for optional arguments
+  !! to subprograms. If `x` is an `optional` parameter of a
+  !! subprogram, then the expression `default(x, y)` inside that
+  !! subprogram evaluates to `x` if it is present, otherwise `y`.
+  !!
+  !! It is an error to call `default` with a single actual argument.
+  !!
+  !! For additional clarity, `default` be called with keyword argument
+  !! for the fallback value, e.g., `default(x, to=1.0)`.
+  !!
+  use iso_fortran_env, only: sp => real32, dp => real64, qp => real128, int16, int32, int64
+  implicit none
+
+
+  private
+  public :: default
+
+
+  interface default
+     module procedure default_sp
+     module procedure default_dp
+     module procedure default_qp
+     module procedure default_int16
+     module procedure default_int32
+     module procedure default_int64
+     module procedure default_logical
+     module procedure default_character
+     ! TODO: complex kinds
+     ! TODO: differentiate ascii & ucs char kinds
+  end interface default
+
+
+contains
+
+
+  function default_sp(x, to) result(y)
+    real(sp), intent(in), optional :: x
+    real(sp), intent(in) :: to
+    real(sp) :: y
+
+    if (present(x)) then
+       y = x
+    else
+       y = to
+    end if
+  end function default_sp
+
+
+  function default_dp(x, to) result(y)
+    real(dp), intent(in), optional :: x
+    real(dp), intent(in) :: to
+    real(dp) :: y
+
+    if (present(x)) then
+       y = x
+    else
+       y = to
+    end if
+  end function default_dp
+
+
+  function default_qp(x, to) result(y)
+    real(qp), intent(in), optional :: x
+    real(qp), intent(in) :: to
+    real(qp) :: y
+
+    if (present(x)) then
+       y = x
+    else
+       y = to
+    end if
+  end function default_qp
+
+
+  function default_int16(x, to) result(y)
+    integer(int16), intent(in), optional :: x
+    integer(int16), intent(in) :: to
+    integer(int16) :: y
+
+    if (present(x)) then
+       y = x
+    else
+       y = to
+    end if
+  end function default_int16
+
+
+  function default_int32(x, to) result(y)
+    integer(int32), intent(in), optional :: x
+    integer(int32), intent(in) :: to
+    integer(int32) :: y
+
+    if (present(x)) then
+       y = x
+    else
+       y = to
+    end if
+  end function default_int32
+
+
+  function default_int64(x, to) result(y)
+    integer(int64), intent(in), optional :: x
+    integer(int64), intent(in) :: to
+    integer(int64) :: y
+
+    if (present(x)) then
+       y = x
+    else
+       y = to
+    end if
+  end function default_int64
+
+
+  function default_logical(x, to) result(y)
+    logical, intent(in), optional :: x
+    logical, intent(in) :: to
+    logical :: y
+
+    if (present(x)) then
+       y = x
+    else
+       y = to
+    end if
+  end function default_logical
+
+
+  function default_character(x, to) result(y)
+    character(len=*), intent(in), optional :: x
+    character(len=*), intent(in) :: to
+    character(len=:), allocatable :: y
+
+    if (present(x)) then
+       y = x
+    else
+       y = to
+    end if
+  end function default_character  
+
+end module default_m