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Update the motivating example to match actual motivating example

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Vyacheslav Egorov 2021-08-06 21:11:08 +03:00
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23
rfcs/syntax-default-type-alias-type-parameters.md
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@ -9,7 +9,20 @@ Introduce syntax to provide default type values inside the type alias type param
Luau has support for type parameters for type aliases and functions.
In languages with similar features like C++ and TypeScript, it is possible to specify default values and users with experience in those languages would like to use that in Luau.
As an example, this feature can be used to define a equality comparator function type that by default accepts a single type argument and assigns the same type to the second argument, but it's still possible to specify both type arguments:
Here is an example that is coming up frequently during development of GraphQL Luau library:
```lua
export type GraphQLFieldResolver<
TSource,
TContext,
TArgs = { [string]: any }
> = (TSource, TArgs, TContext, GraphQLResolveInfo) -> any
```
If we could specify defaults like that, we won't have to write long type names when type alias is used unless specific customization is required.
Some engineers already skip these extra arguments and use `any` to save time, which gives worse typechecking quality.
Without default arguments it's also harder to refactor the code as each type alias reference that uses 'common' type arguments has to be updated.
While previous example uses a concrete type for default type value, it should also be possible to reference generic types from the same list:
```lua
type Eq<T, U = T> = (l: T, r: U) -> boolean
@ -17,14 +30,6 @@ local a: Eq<number> = ...
local b: Eq<number, string> = ...
```
While previous examples reference type parameters from the same list, it is also possible to specify concrete types:
```lua
type StrArray<T = string> = {arr: {T}, locked: boolean}
local a: StrArray = ...
local b: StrArray<MyString> = ...
```
Generic functions in Luau also have a type parameter list, but it's not possible to specify type arguments at the call site and because of that, default type argument values for generic functions are not proposed.
## Design