On 2/16/2022 2:43 PM, Ian Lance Taylor wrote:
On Tue, Feb 15, 2022 at 2:49 PM Blackgreen<bg.theroc...@gmail.com> wrote:
I came across this stack overflow question:
https://stackoverflow.com/questions/71131665/generics-pass-map-with-derived-types
The OP attempted to pass different maps into this code:
func equal[M1, M2 ~map[K]V, K, V comparable](m1 M1, m2 M2) bool { if len(m1) !=
len(m2) { return false } for k, v1 := range m1 { if v2, ok := m2[k]; !ok || v1
!= v2 { return false } } return true }
It is clear to me why this doesn't work. However, I was studying possible fixes
for this function. Here's a
playground:https://gotipplay.golang.org/p/NPpau1G0Hqp
My first attempt was based on differentiating the K and V type params and came
up with this:
func equal[M1 ~map[K1]V1, M2 ~map[K2]V2, K1, K2 ~uint32, V1, V2 ~string](m1 M1,
m2 M2) bool {
if len(m1) != len(m2) {
return false
}
for k, v1 := range m1 {
if v2, ok := m2[K2(k)]; !ok || V2(v1) != v2 {
return false
}
}
return true
}
I thought this should work, because now it's possible to infer K1 and K2 from M1 and M2
respectively, and the approximate constraints allow conversion in the function body. But
it still doesn't compile with error "K2 does not match uint32".
Instead, this works, even though K1 and K2 are defined the same way:
func equalFixed[K1, K2 ~uint32, V1, V2 ~string](m1 map[K1]V1, m2 map[K2]V2)
bool {
}
By reading the Go 1.18 language specs, my intuition is that the first attempt fails due
to the so-called "adjusted core type" of the constraint ~uint32, though I'm not
100% sure of what's going on.
Can you folks confirm my intuition and/or provide some pointers?
This looks like a bug in type inference to me. I may be missing
something but I don't see how it could be producing that error. Would
you mind opening a bug report athttps://go.dev/issue? Thanks. Note
that this may not be fixed in the upcoming 1.18 release, as we are
trying to be careful about type inference.
Ian
I changed the block below to use alias declarations instead and it
worked OK for me:
type (
someNumericID uint32
someStringID string
)
to this:
type (
someNumericID = uint32
someStringID = string
)
https://gotipplay.golang.org/p/diU2c31TC1b
The Go Programming Language Specification - Go 1.18 Draft - The Go
Programming Language (golang.org)
<https://tip.golang.org/ref/spec#Alias_declarations>
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