implicit none (Fortran)

Applies to: fortran

implicit none turns off Fortran's old rule that undeclared names are auto-typed by their first letter. It forces you to declare every variable, turning typos into compile errors instead of silent bugs. Always include it.

subroutine f()
  implicit none
  real :: x      ! must be declared
  x = 1.0
end subroutine

See also: module, data-type