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doc: explain current limitations of generics (#6674)

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Nick Treleaven 2020-10-24 18:37:14 +01:00 committed by GitHub
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1 changed files with 23 additions and 14 deletions
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@ -1800,15 +1800,21 @@ fn (r Repo<T>) find_by_id(id int) ?T {
} }
db := new_db() db := new_db()
users_repo := new_repo<User>(db) users_repo := new_repo<User>(db) // returns Repo<User>
posts_repo := new_repo<Post>(db) posts_repo := new_repo<Post>(db) // returns Repo<Post>
user := users_repo.find_by_id(1)? user := users_repo.find_by_id(1)? // find_by_id<User>
post := posts_repo.find_by_id(1)? post := posts_repo.find_by_id(1)? // find_by_id<Post>
``` ```
At the moment only one type parameter named `T` is supported.
Currently generic function definitions must declare their type parameters, but in
future V will infer generic type parameters from single-letter type names in
runtime parameter types. This is why `find_by_id` can omit `<T>`, because the
receiver argument `r` uses a generic type `T`.
Another example: Another example:
```v ```v
fn compare<T>(a, b T) int { fn compare<T>(a T, b T) int {
if a < b { if a < b {
return -1 return -1
} }
@ -1818,17 +1824,20 @@ fn compare<T>(a, b T) int {
return 0 return 0
} }
println(compare<int>(1,0)) // Outputs: 1 // compare<int>
println(compare<int>(1,1)) // 0 println(compare(1, 0)) // Outputs: 1
println(compare<int>(1,2)) // -1 println(compare(1, 1)) // 0
println(compare(1, 2)) // -1
println(compare<string>('1','0')) // Outputs: 1 // compare<string>
println(compare<string>('1','1')) // 0 println(compare('1', '0')) // Outputs: 1
println(compare<string>('1','2')) // -1 println(compare('1', '1')) // 0
println(compare('1', '2')) // -1
println(compare<float>(1.1, 1.0)) // Outputs: 1 // compare<f64>
println(compare<float>(1.1, 1.1)) // 0 println(compare(1.1, 1.0)) // Outputs: 1
println(compare<float>(1.1, 1.2)) // -1 println(compare(1.1, 1.1)) // 0
println(compare(1.1, 1.2)) // -1
``` ```