guideline

Use blocks to localize the scope of declarations.

Use blocks to perform local renaming.

Use blocks to define local exception handlers.

with Motion;
with Accelerometer_Device;
...

   ---------------------------------------------------------------------
   function Maximum_Velocity return Motion.Velocity is

      Cumulative : Motion.Velocity := 0.0;

   begin  -- Maximum_Velocity

      -- Initialize the needed devices
      ...

      Calculate_Velocity_From_Sample_Data:
         declare
            use type Motion.Acceleration;

            Current       : Motion.Acceleration := 0.0;
            Time_Delta    : Duration;

         begin  -- Calculate_Velocity_From_Sample_Data
            for I in 1 .. Accelerometer_Device.Sample_Limit loop

               Get_Samples_And_Ignore_Invalid_Data:
                  begin
                     Accelerometer_Device.Get_Value(Current, Time_Delta);
                  exception
                     when Constraint_Error =>
                        null; -- Continue trying

                     when Accelerometer_Device.Failure =>
                        raise Accelerometer_Device_Failed;
                  end Get_Samples_And_Ignore_Invalid_Data;

               exit when Current <= 0.0; -- Slowing down

               Update_Velocity:
                  declare
                     use type Motion.Velocity;
                     use type Motion.Acceleration;

                  begin
                     Cumulative := Cumulative + Current * Time_Delta;

                  exception
                     when Constraint_Error =>
                        raise Maximum_Velocity_Exceeded;
                  end Update_Velocity;

            end loop;
         end Calculate_Velocity_From_Sample_Data;

      return Cumulative;

   end Maximum_Velocity;
   ---------------------------------------------------------------------
...

rationale

Blocks break up large segments of code and isolate details relevant to each subsection of code. Variables that are only used in a particular section of code are clearly visible when a declarative block delineates that code.