The uint8 in cachedFn[uint8, V] is the type parameter name, it can be
arbitrary valid identifier.
It shadows the predeclared uint8 identifier.
On Friday, November 10, 2023 at 12:25:05 PM UTC+8 ahuigo wrote:
> Btw, please ignore the type logic in my code, I wrote this piece of code
> just to illustrate the oddities of generics in struct methods.
> Regardless of whether its type is int, string, any, *or the exact uint8,
> this error is very strange.*
>
>
> // it doesn't work
> func (c *cachedFn[uint8, V]) Get0() (V, error) {
> var s uint8 = 0
> s = 0 // error: cannot use 0 (untyped int constant) as uint8 value in
> assignment
> fmt.Printf("cache key: %#v, %T\n", s, s) // cache key: 0, uint8
> return c.Get(s)
> }
>
> // it works
> func (c *cachedFn[uint8, V]) Get0() (V, error) {
> var s uint8 = 0
> fmt.Printf("cache key: %#v, %T\n", s, s) // cache key: 0, uint8
> return c.Get(s)
> }
> On Friday, November 10, 2023 at 4:34:46 AM UTC+8 Axel Wagner wrote:
>
>> Yes, this has come up before.
>>
>> On Fri, Nov 10, 2023 at 7:09 AM ahuigo <[email protected]> wrote:
>>
>>> There is an example: https://go.dev/play/p/guzOWRKi-yp
>>>
>>> ```
>>> func (c *cachedFn[string, V]) Get0() (V, error) {
>>> // var s any
>>> var s string
>>> s = "abc" // error: cannot use "abc" (untyped string constant) as string
>>> value in assignment
>>> fmt.Printf("cache key: %#v, %T\n", s, s) // cache key: 0, uint8
>>> return c.Get(s)
>>> }
>>> ```
>>> I find the generic type of the struct method a bit confusing.
>>> 1. The type `cachedFn[string, V]` does not really constrain the type
>>> of `s` to **string**. It's actual type is `uint8`
>>>
>>
>> The type `cachedVn[string, V]` *would* in fact instantiate `cachedVn`
>> with `string` and `V`.
>> But that's not what you are doing. You are writing the receiver type as
>> `fun c(c *cachedFn[string, V])`, which means that "the receiver is the
>> generic type `cachedVn` with two type parameters called `string` and `V`".
>> Predeclared identifiers in Go are not special in any way, you can re-use
>> them for your own variables and types - or type parameters. So what you are
>> doing here is fundamentally similar to this problem:
>> https://go.dev/play/p/lDE-o7fGHi8
>>
>> There probably should be a vet check for using a predeclared identifier
>> as a type parameter name (or maybe even for any re-use of a predeclared
>> identifier).
>>
>> 2. And this error is a bit strange. (`s = "abc" // error: cannot use
>>> "abc" (untyped string constant) as string value in assignment. ` )
>>>
>>>
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