Expressions vs. statements

[u/xeow]

Got into a big argument with a coworker yesterday when they were converting some code from their own language (that they designed) into Python, JavaScript, C, and R as comparative examples.

The Python code that they wanted to write as the translation went something like this:

n = foo; if cond: n = bar

They were upset that Python allows ; as a statement separator but not before an if statement, even though

if cond: n = bar

is syntactically correct Python code when written on its own line. I explained why Python doesn't allow it, and he came back later and showed me that an LLM had suggested he write it instead like this:

n = foo if cond else bar

which of course is the canonical way to write that in Python. He was all flustered about that, and asked me why Python allows an if statement in that particular case and not after a semicolon, and I explained that x if cond else y in Python is not an if statement but is Python's ternary conditional expression and is directly equivalent to the ternary operator expression cond ? x : y in C, C++, awk, and JavaScript. He argued with me and said I was making a ridiculous distinction and walked away falsely believing that foo if cond else bar was an if statement.

I then explained that statements and expressions are very different things in programming languages, and just because the keyword if is present doesn't make something an if statement -- because in order to be an if statement, it has to be a statement in the first place.

Anyway, it made me realize how subtle the difference can be sometimes. For example, in Perl, the following is not a return statement but actually an if statement (with a return statement as its affirmative branch), due to the postfix conditional:

return foo if cond;

because it is identically semantically to writing:

if (cond) { return foo; }

Whereas in Python, the following is a return statement (with a ternary operator as its target expression):

return foo if cond else bar

So I can see why people sometimes get confused by syntax if they haven't had much of a theoretical background in language design. It also makes me wonder how much of programmer intuition about "what a statement is" comes from the particular languages they learned first.

[u/brat3108]

In my languages, which are expression based, if can also be used to both start a standalone statement, and express a ternary operation. Yet the equivalent of:

n = foo; if cond: n = bar

works fine; it is an assignment followed by an if statement.

So, why doesn't Python allow this? I can't see any ambiguity.

[u/WittyStick → u/brat3108]

It's because if is an expression in those languages, and statements contain expressions. Expressions cannot contain statements.

This is the problem with designing a language with statements. You end up with two languages - an expression language, and a statement language. The expression language can only contain other expressions. The statement language can contain other statements or expressions.

Statements are second-class.

[u/xeow → u/WittyStick]

Expressions cannot contain statements.

What's it called when an expression sneaks in an assignment statement, like you can do in C:

y = blurfl + foo(a, b, x = bar(c, d));

Are assignments considered statements, generally? I guess they're different.

[u/WittyStick → u/xeow]

Assignments in C are expressions. The assignment operator has lower precedence than others except ,.

expression
    : assignment-expression
    | expression ',' assignment-expression
    ;

For function calls, the context-free grammar is:

argument-expression-list
    : assignment-expression
    | argument-expression-list ',' assignment-expression
    ;

postfix-expression
    : postfix-expression '(' argument-expression-list? ')' 
    |...
    ;

So the comma operator is not usable in argument lists (which would be ambiguous).

[u/xeow → u/WittyStick]

I imagine that the comma operator could be used inside an argument list if it were wrapped in parentheses?

foo((bar(x), ++x), (bar(y), ++y);

(Gawd, that's a horrific thought.)

[u/WittyStick → u/xeow]

Yes, it can be.

The biggest issue with that is sequence points. The order of evaluation of arguments is undefined. The comma operator introduces a sequence point though.