Python Comprehensions in Cpp2

[msadeqhe]

Let's again glance terser function syntax:

: (x) x + 1

Cpp2 already doesn't require parentheses within control structures, and it allows to write expressions without = in function declarations at the same time. These syntax choices made Cpp2's syntax somehow compatible with Python's if-expressions and Comprehensions. What if Cpp2 also support them?

x if x < 10 else x + 1
i for i in list if i < 10

For example:

var1: i32 = 0 if is_off() else 1;

list: vector<i32> = (1, 2, 3, 4, 5);
var2: = i * 2 for i in list; // (2, 4, 6, 8, 10)

Comprehensions are harder to implement than if-expressions. So it may be the reason to completely ignore them in Cpp2.

[AbhinavK00]

Comprehensions would eliminate the need for the ternary ?: operator. And that would unify cpp's if statements and ternary expressions under one if in cpp2.

[hsutter]

Re ternary ?: operator: Yes, my thought has been that if I support that I would try if-expressions with required () around the alternative expressions. For example: y = if x<10 (x-1) else (x*42); That's symmetric with if-statements with required {} around the alternative statements.

[msadeqhe]

Yes. Python-style is more expressive unless consistency with Cpp2 syntax is a concern.

[msadeqhe]

Alternative syntax with ?:

a: = if x < 10 ? x else x - 10;
b: = for list do (i) i * 2;

Also do (i) i * 2 is similar to terser lambda syntax but it's without :.

[AbhinavK00]

Using a full character for a small feature isn't a good idea I think.

[msadeqhe]

You're right, but ? can be extended to other control structures too:

a: = while x < 10 next i++ ? i * 2;

BTW ? is already used in Cpp1. It's just another alternative syntax.

[msadeqhe]

Using () in if-expressions with direct and list initializations:

v32: type == std::vector<i32>;

// With required `()`
var1: v32 = if x < 10 ((1, 2)) else ((1, 2, 3));

// With optional `()`
var2: v32 = if x < 10 (1, 2) else (1, 2, 3);

[jcanizales]

What would the type of var2 be?

[msadeqhe]

Its return type would be std::generator<i32>. It's like writing a coroutine function with co_yield.

[msadeqhe]

• Discussion Block Expressions with Consistent Syntax to Declarations #836 is a generalized feature based on issue [BUG] == and = in declarations are confusing. #824 and the idea of @ntrel to use = in control flows. I close this discussion in favor of block expressions. Thanks everyone.