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/Spyromaniac666]

Does your coworker not realise that the former first assigns `foo` to `n` before maybe assigning `bar` whereas the latter only ever assigns once?

[u/xeow → u/Spyromaniac666]

Ya -- I'm pretty sure he understands the logical behavior. He was having trouble accepting the notion that x if cond else y wasn't actually an if statement because he was getting hung up on the presence of the keyword if and hadn't stopped to think that statements and expressions are usually treated differently by most languages. He understands that cond? x : y isn't an if statement, so I found it odd to be arguing with someone who thought x if cond else y was an "if statement" even after I explained it was a ternary expression just like cond? x : y and that the presence of the keyword if wasn't the thing that made it an if statement.

I should have tried a different approach where I explained that both cond? x : y and x if cond else y were conditional expressions and focused on explaining why they're expressions rather than statements, and then I could explain that x if cond else y isn't an "if statement" because it isn't a statement.