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?
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