the type `null | true | false` is different from `true | false`, a type checker
can assert that you handle the `null` case before using a function that wants a boolean.
This is how rust handles it (with the Option<T> type).
That doesn’t make sense. `null | false | true` is not equivalent to `Option<bool>` or `Option<false | true>`. Just like `zero | one | two` is not equivalent to `Option<one | two>`.
Assuming that `true | false` is equivalent to something like `enum { true, false }`.
Note that 3VL (3 valued logic) is one of the more criticized aspects of SQL.
3VL is a gigantic leap of of complexity over 2VL (boolean). The number of possible operations is a lot higher and the choice of operations is not standardized. To quote wikipedia: "In two-valued logic there are 2 nullary operators (constants), 4 unary operators, 16 binary operators, 256 ternary operators" And we all agree which of those 16 is named what. "In three-valued logic there are 3 nullary operators (constants), 27 unary operators, 19683 binary operators, 7625597484987 ternary operators" There is no standard meaning of the 3'rd value and of the operations involving it.
That is not to say that 3VL isn't useful. I believe the reason for the criticism of 3VL in SQL it that the system made those choices for you and those choices are not always intuitive. Therefore a null in one attribute in one relation might mean something very different from a null in another attribute in another relation.
Also note that as opposed to digital signal processing (where binary and ternary refer to the number of possible values a bit/trit can have), when discussing logic arity (nullary, unary, binary, ternary, n-ary) refers to the number of arguments required by a function as opposed to the number of values the arguments can have (univalent, bivalent, trivalent, k-valent). This is confusing because the terms are used interchangeably.
avgcorrection|3 years ago
Assuming that `true | false` is equivalent to something like `enum { true, false }`.
evomassiny|3 years ago
Isn't it ? both cases represent a type than can express 3 variants
harperlee|3 years ago
alexvoda|3 years ago
3VL is a gigantic leap of of complexity over 2VL (boolean). The number of possible operations is a lot higher and the choice of operations is not standardized. To quote wikipedia: "In two-valued logic there are 2 nullary operators (constants), 4 unary operators, 16 binary operators, 256 ternary operators" And we all agree which of those 16 is named what. "In three-valued logic there are 3 nullary operators (constants), 27 unary operators, 19683 binary operators, 7625597484987 ternary operators" There is no standard meaning of the 3'rd value and of the operations involving it.
That is not to say that 3VL isn't useful. I believe the reason for the criticism of 3VL in SQL it that the system made those choices for you and those choices are not always intuitive. Therefore a null in one attribute in one relation might mean something very different from a null in another attribute in another relation.
Also note that as opposed to digital signal processing (where binary and ternary refer to the number of possible values a bit/trit can have), when discussing logic arity (nullary, unary, binary, ternary, n-ary) refers to the number of arguments required by a function as opposed to the number of values the arguments can have (univalent, bivalent, trivalent, k-valent). This is confusing because the terms are used interchangeably.
freilanzer|3 years ago
evomassiny|3 years ago