| cue eval coalesce.cue |
| cmp stdout expect-stdout-cue |
| |
| -- frontmatter.toml -- |
| title = "Null Coalescing" |
| description = "" |
| |
| -- text.md -- |
| <!-- jba: the terms here are confusing. "Null coalescing" is actually not |
| that, but then there is something called "actual null coalescing." |
| |
| Just say that because _|_ | X evaluates to X, you can use disjunction |
| to represent fallback values. |
| |
| And then you can use that to effectively type-check with a default value. |
| --> |
| |
| With null coalescing we really mean error, or bottom, coalescing. |
| The defaults mechanism for disjunctions can also be |
| used to provide fallback values in case an expression evaluates to bottom. |
| |
| In the example the fallback values are specified |
| for `a` and `b` in case the list index is out of bounds. |
| |
| To do actual null coalescing one can unify a result with the desired type |
| to force an error. |
| In that case the default will be used if either the lookup fails or |
| the result is not of the desired type. |
| |
| -- coalesce.cue -- |
| list: [ "Cat", "Mouse", "Dog" ] |
| |
| a: *list[0] | "None" |
| b: *list[5] | "None" |
| |
| n: [null] |
| v: *n[0]&string | "default" |
| |
| -- expect-stdout-cue -- |
| list: ["Cat", "Mouse", "Dog"] |
| a: "Cat" |
| b: "None" |
| n: [null] |
| v: "default" |
