No warning when assiging to a variable

mikeraz has asked for the wisdom of the Perl Monks concerning the following question:

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Re: No warning when assiging to a variable by vsespb (Chaplain) on Aug 14, 2013 at 18:49 UTC
It's the same kind of typo error that is being caught with $foo = 2 it's not the same. `if ($foo = $bar)` [download] is same as `$foo = $bar; if ($bar)` [download] which makes some sense. while `if ($foo = 2)` [download] is same as `$foo = 2 if (2)` [download] which does not make much sense.	[reply] [d/l] [select]
Re: No warning when assiging to a variable by McA (Priest) on Aug 14, 2013 at 18:52 UTC
Hi, just a guess: At compile time the compiler knows that the expression in the if statement will always evaluate to a constant 2, the if clause is probably meaningless, you get a warning. In the case of `if ($foo = $bar)` it may be intended, but anyway: The compiler can't evaluate the if clause to a constant as $bar can be true or false, so the conditional `if` may make sense. Best regards McA	[reply] [d/l] [select]
Re^2: No warning when assiging to a variable by afoken (Chancellor) on Aug 16, 2013 at 19:38 UTC
Let's have a look at what gcc does: /tmp>cat gcc-warn-assign.c int main(void) { int i,x,y; i=x=y=0; /* unintentional assignment inside if condition / if (x=42) i++; if (x=y) i++; / intentional assignment inside if condition, marked with ext +ra parentheses / if ((x=42)) i++; if ((x=y)) i++; return i; } /tmp>gcc -Wall -pedantic gcc-warn-assign.c gcc-warn-assign.c: In function ‘main’: gcc-warn-assign.c:6:2: warning: suggest parentheses around assignment +used as truth value gcc-warn-assign.c:7:2: warning: suggest parentheses around assignment +used as truth value /tmp>gcc --version gcc (GCC) 4.5.2 Copyright (C) 2010 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There i +s NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PUR +POSE. /tmp> [download] So, what do we see here? (Ignore `i` and `i++`, it's just dummy code.) Possibly unintentional assignments inside `if` conditions cause warnings, no matter what is on the RHS of the assignment operator. The RHS may be a constant, or any other expression. It does not matter, and it should not* matter. If you want to write "especially clever" code, you still can assign inside an `if` condition without warnings, but you must wrap the assignment in semi-redundant parentheses to make your intention -- you want to test the truth of the result of the assignment -- clear to gcc. And the code is still compatible with other C compilers that do not know the extra parentheses trick, they just ignore the redundant parentheses. I think Perl with `warnings` enabled should behave the same: Warn if a variable is assigned anything, constant or not, inside the condition of `if` and friends, because it is likely a bug. And don't warn only if the assignment has extra parentheses indicating someone wanted to write "especially clever" code. Alexander -- Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)	[reply] [d/l] [select]
Re^3: No warning when assiging to a variable by McA (Priest) on Aug 16, 2013 at 20:46 UTC
Hi Alexander, Perl is not C. Let's look at a common idiom in Perl: `while(my $line = <>) { do_something($line); }` [download] This is an assignment in a conditional operator. It does fortunately not emit a warning. Let's look at a more or less equivalent code in C: `#include <stdio.h> #include <stdlib.h> int get_val(void); int main(void) { int i, x, y; i = x = y = 0; while (x = get_val()) { i++; printf("in loop\n"); if(i == 1) { exit(1); } } return i; } int get_val(void) { return(1); }` [download] Compiled with `LANG=C gcc -Wall -pedantic -o warning warning.c` it does emit the following: `warning.c: In function 'main': warning.c:11: warning: suggest parentheses around assignment used as t +ruth value` [download] IMHO, it proves: Perl is not C, C is not Perl. C is a compiled language. Perl is a interpreted language. In my opinion, the above Perl idiom should not emit a warning like C does. Best regards McA P.S.: A ++ for your comparison.	[reply] [d/l] [select]
Re^4: No warning when assiging to a variable by afoken (Chancellor) on Aug 20, 2013 at 18:54 UTC
Re: No warning when assiging to a variable by code-ninja (Scribe) on Aug 15, 2013 at 05:23 UTC
there is no warning because there is actually nothing to warn about. when you say `$foo = $bar` you're just assigning the value and there will be no conditions to evaluate. Inside an `if' conditional, the value assigned to the LHS will be used as is, like `$bar = 2; if($foo = $bar) { # do something }` [download] is same as `if(2){ # do something }` [download]	[reply] [d/l] [select]
Re: No warning when assiging to a variable by locked_user sundialsvc4 (Abbot) on Aug 14, 2013 at 19:20 UTC
What you are asking for is a “perl `lint`,” which catches the stuff that is “technically legal but probably not what you wanted to do,” and I do not readily know of one. Is there one? `Q:` Why do computer programmers have flat heads? `A:` (Slap!) “D-oh!”
Re^2: No warning when assiging to a variable by GotToBTru (Prior) on Aug 14, 2013 at 21:19 UTC
Perl Critic, I suppose, is supposed to be the perl lint. It says, on brutal mode: No package-scoped "$VERSION" variable found Return value of flagged function ignored - print Return value of flagged function ignored - print	[reply]