GLSL for loop condition expression: how can it be a declaration?

[andy3]

The spec says a for loop is:

for (init-expression; condition-expression; loop-expression)

And then they say:

Expressions for condition-expression must evaluate to a Boolean.
Both the condition-expression and the init-expression can declare and initialize a variable

And the grammar has:

condition :
    expression
    fully_specified_type IDENTIFIER EQUAL initializer

I’m confused about to use this condition expression variable initialization case. I’ve tried this in WebGL 1 GLSL:

for(int i = 0; bool x = i < 10; i++) {}

And the angle compiler errors and tells me me:

Expected loop index

Can someone provide valid syntax for declaring a variable in the condition expression? Also, in GLSL, do declarations evaluate as expressions? Aka does bool x = false; as a statement also evaluate to false if used in the context of an expression?

I’m working in WebGL, GLSL ES 1.0, and I wonder if this is GLSL ES 3.0 only, or if I’m missing something.

[Alfonse_Reinheart]

The spec (at least GLSL 4.6) talks about this, but within a context where it’s talking about all of these:

for (init-expression; condition-expression; loop-expression)
    sub-statement
while (condition-expression)
    sub-statement
do
    statement
while (condition-expression)

It is possible that it is only talking about the “condition-expression” term of while (and not the do-while version either). Then again, “condition-expression” is not a valid term in the grammar.

Alternatively, in C and C++, definitions are legal in the condition clause of a for loop, but only really because for is explicitly defined in terms of while.

Can someone provide valid syntax for declaring a variable in the condition expression?

I wouldn’t take Angle’s word for it with regard to what is and is not valid GLSL grammar. Assuming “condition-expression” is a condition term, then the code you wrote is valid.

[andy3]

I was able to verify that this compiles in WebGL2 / GLSL ES 3.0:

for(int i = 0; bool x = false; i++) {}

So looks like it’s just not available syntax in GLSL ES 1.0. I’m still confused on if that whole declaration returns the value false, or if it’s some magic specific handling of the condition-expression by compilers.

[GClements]

@andy3

So looks like it’s just not available syntax in GLSL ES 1.0.

According to appendix A:

Appendix A: Limitations for ES 2.0

1 Overview

OpenGL ES 2.0 implementations are not required to support the full GLSL ES 1.00 specification. This section lists the features which are not fully supported in ES 2.0. Features not listed in this section must be supported in their entirety.
…
4 Control Flow
…
for loops are supported but with the following restrictions:
…

• condition has the form
  loop_index relational_operator constant_expression
  where relational_operator is one of: > >= < <= == or !=

Ultimately, there’s not much point in using a declaration in a condition expression, as the body will never be executed with the variable having a value other than true. Section 10 (Issues) has this to say:

10.20 While-loop Declarations

What is the purpose of allowing variable declarations in a while statement?

while (bool b = f()) {...}

Boolean b will always be true until the point where it is destroyed. It is useful in C++ since integers are implicitly converted to booleans.

RESOLUTION: Keep this behavior. Will be required if implicit type conversion is added to a future version.

With implicit type conversion, you could have

for (...; int x=f(); ...)

with the loop body using the value of x (which could be any non-zero value).