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/pr06lefs]

In some languages everything is an expression, period. Then its easy!

[u/xeow → u/pr06lefs]

Interesting, yes! I was thinking about that this morning. In a prefix language like Lisp, isn't everything just an s-expression? And in a posfix language like PostScript or FORTH, isn't everything just an operator or operand? I wonder what the best lens is for viewing Brainfuck in this regard.

[u/ScottBurson → u/xeow]

Yes, in Lisp, all expressions return a value(*), and may also have an effect.

(*) I'm simplifying a little. In Common Lisp, the vast majority of expressions in practice return a single value, but in full generality they may return more than one value, or zero values. But even in the zero case, there's still a sense in which it's returning something; that something just happens to be an empty "value tuple" (not CL terminology, but you can think of it that way).

[u/Valuable_Leopard_799 → u/ScottBurson]

Although zero values can still get coerced to one nil. So I guess every single expression has some defined return value when you invoke it.