3 ms·

A predicate?

tialaramex 4y ago
No, if, and match, and most things are expressions, and thus have a (typed) value. In a typical programming language we can write something like x = 5; assigning 5 to the variable x. That 5 there is an expression, we can write x = (3 * 2) - 1; with the same effect, that's a more complicated expression. We can call a function here: x = f(3); or if our language has methods we can even call those: x = some.function(parameters); In Rust I can write x = if it.should_be_nine() { 9 } else { 5 }; // because if is an expression Since Rust's loops are also expressions, x = loop { let stuff = get_stuff(); if stuff.is_good() { break stuff } }; // ... once the stuff is good, break out of the loop and we'll assign that stuff to x
- smitty1e 4y ago
  Understood, these things are expressions, but must they nit evaluate to a boolean in order for if/switch/match to branch correctly? My informal understanding of a predicate may need refining.
  - tialaramex 4y ago
    I think the thing you're missing is that in some languages we're allowed to use these constructs as expressions. Not that they take expressions which is common in modern non-toy languages, but that they themselves are expressions. It might be you've just always used languages where you can do this, or that you assumed you can but actually you never tried in a language you use. In C for example we can't write this: int x = if (1 == 2) { 3 } else { 4 }; // Not valid C syntax C provides a special operator, the only one in the language which has three operands and thus it's often just called "the ternary operator" although other ternary operators can technically exist, to achieve the same thing: int x = (1 == 2) ? 3 : 4; // Valid C syntax using ternary operator But, how about this? int x = switch 3 { case 1: 4; case 2: 8; default: 0; }; // Can't do this either There's no way to express that in C or C++ today. The closest is the "Immediately Invoked Lambda" from C++ which is illustrated nearby in this thread. Barry Revzin apparently proposed a way to do this in C++ 26 because C++ wants to have pattern matching and pattern matching kinda sucks without this feature.

A predicate?

No, if, and match, and most things are expressions, and thus have a (typed) value. In a typical programming language we can write something like x = 5; assigning 5 to the variable x. That 5 there is an expression, we can write x = (3 * 2) - 1; with the same effect, that's a more complicated expression. We can call a function here: x = f(3); or if our language has methods we can even call those: x = some.function(parameters); In Rust I can write x = if it.should_be_nine() { 9 } else { 5 }; // because if is an expression Since Rust's loops are also expressions, x = loop { let stuff = get_stuff(); if stuff.is_good() { break stuff } }; // ... once the stuff is good, break out of the loop and we'll assign that stuff to x

smitty1e 4y ago
Understood, these things are expressions, but must they nit evaluate to a boolean in order for if/switch/match to branch correctly? My informal understanding of a predicate may need refining.
- tialaramex 4y ago
  I think the thing you're missing is that in some languages we're allowed to use these constructs as expressions. Not that they take expressions which is common in modern non-toy languages, but that they themselves are expressions. It might be you've just always used languages where you can do this, or that you assumed you can but actually you never tried in a language you use. In C for example we can't write this: int x = if (1 == 2) { 3 } else { 4 }; // Not valid C syntax C provides a special operator, the only one in the language which has three operands and thus it's often just called "the ternary operator" although other ternary operators can technically exist, to achieve the same thing: int x = (1 == 2) ? 3 : 4; // Valid C syntax using ternary operator But, how about this? int x = switch 3 { case 1: 4; case 2: 8; default: 0; }; // Can't do this either There's no way to express that in C or C++ today. The closest is the "Immediately Invoked Lambda" from C++ which is illustrated nearby in this thread. Barry Revzin apparently proposed a way to do this in C++ 26 because C++ wants to have pattern matching and pattern matching kinda sucks without this feature.