Ada and goto

By: spqr

1 March 2019 at 16:10

Every language has its hidden “features”, and Ada, like most other languages has a goto statement. Any statement could be labelled by an identifier enclosed in double angle brackets, << >>. For example:

<<gohere>> g = 12.3; ...
goto gohere;

However, the goto in Ada is somewhat better behaved. It cannot transfer control outside the current subprogram or package body; it cannot transfer control inside a structure (e.g. from else to then in an if statement); and it cannot transfer control from the outside of a structured statement into the body of a structured statement. Consider the following code:

with ada.Text_IO; use Ada.Text_IO;
with ada.Integer_Text_IO; use ada.Integer_Text_IO;

procedure gotoada is
   x,y,z : integer;
 
begin
         put_line("Enter a number: ");
         get(x);
<<lbl1>> if x = 1 then
            y := 1;
            z := 1;
<<lbl2>> else
            y := 2;
            z := 2;
         end if;

         case y is
            when 1 =>
               put_line("jump to top");
               goto lbl1;
            when others =>
               put_line("jump to middle");
               goto lbl2;
         end case;

end gotoada;

The goto associated with lbl1 is allowed. However the goto associated with lbl2, which jumps into the middle of the if statement above it, is not allowed. Try to compile this, and you’ll get the following compiler message:

gotoada.adb:24:21: target of goto statement is not reachable

Languages like Pascal would easily allow this sort of code, but Ada was designed to prevent issues with uber unstructured code.

spqr

💾

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