kst@telesoft.UUCP (Keith Thompson @diego) (11/09/90)
> The Question: > Why does this procedure raise constraint error? > It happens on the second assignment to dummy, but > doesn't happen if dummy.len > 0 and appears to have > something to do with the function call "&" Yes, this is pretty obscure. Here's what's happening. The lower bound of the array type Data is Short'First, which is 0. The lower bound of Dummy.Text is declared as 1. After the first assignment, Dummy.Text is a null array. By LRM 4.5.3:4, the lower bound of the result of a catenation is the lower bound of the left operand, *unless* the left operand is a null array; in that case, the result is the right operand. By LRM 4.5.3:5, an operand which is of the component type of the array type is treated as a single-element array whose lower bound is the lower bound of the index subtype of the array type (in this case, 0). Since implicit array subtype conversion is done for array assignments, but not for component associations, the association text => dummy.text & 'a' raises Constraint_Error, since the component's lower bound of 1 doesn't match the expression's lower bound of 0. Here's a modified version of the test program that doesn't raise Constraint_Error. Added lines are marked with "--+", changed lines with "--!". with text_io; use text_io; procedure bozo is type short is range 0 .. 10; type data is array (short range <>) of character; type var_text (len : short := 0) is record text : data (1 .. len); end record; dummy : var_text; -- unconstrained procedure do_nothing (d : data) is begin null; end do_nothing; function shift(text: data) return data is --+ result: data(1 .. text'length); --+ begin --+ result := text; --+ return result; --+ end shift; --+ --+ begin put_line ("before"); dummy := (len => 0, text => ""); put_line ("during"); dummy := (len => dummy.len + 1, text => shift(dummy.text & 'a')); --! put_line ("after"); exception when constraint_error => put_line ("constraint_error raised"); end bozo; Here's the output: before during after -- Keith Thompson (The_Other_Keith) ucsd.ucsd.edu!telesoft!kst TeleSoft kst@telesoft.com 5959 Cornerstone Court West San Diego, CA 92121-9891 (619) 457-2700 x121 "hello?"