feat: reformulate the simple assertions

demonstrate `assert` execution in a pure procedure

Author: Damian Rouson

example/simple_assertions.f90

@@ -1,10 +1,33 @@
 program assertion_examples
-  !! Demonstrate the use of assetions
-  use assert_m, only : assert
+  !! Demonstrate the use of assertions as runtime checks on the satisfaction of 
+  !! of two kinds of constraints:
+  !! 1. Preconditions: requirements for correct execution at the start of a procedure and
+  !! 2. Postconditions: requirements for correct execution at the end of a procedure.
   implicit none
-  integer, parameter :: i = 1
-  real, parameter :: x = -1.
 
-  call assert(i > 0, "i > 1") ! Passes
-  call assert(x > 0, description="x > 0", diagnostic_data=x) ! Fails with output containing diagnostic data
+  print *, reciprocal(2.)
+
+contains
+
+  pure real function reciprocal(x) result(reciprocal_of_x)
+    !! Erroneous calculation of the reciprocal of the function's argument
+    use assert_m, only : assert
+    real, intent(in) :: x
+
+    call assert(assertion = x /= 0., description = "reciprocal: x /= 0", diagnostic_data = x) ! Precondition passes
+
+    reciprocal_of_x = 0. ! incorrect value for the reciprocal of x
+
+    block
+      real, parameter :: tolerance = 1.E-06
+
+      associate(error => x*reciprocal_of_x - 1.) 
+
+        call assert(abs(error) < tolerance, "reciprocal: abs(error) < tolerance", error) ! Postcondition fails
+
+      end associate
+    end block
+
+  end function
+
 end program