Because nil is a special thing in Go.

nil is not a value, it's a predeclared identifier.

it represents zero value for pointers, interfaces, maps, slices, channels and function types, representing an uninitialized value

len(nil) feels like it should work if you think of nil as the same as "value of empty array"

but what should be:

  var p *Struct
  len(p) 
  ???

There's no "length of zero-valued pointer".

Because the bare value `nil` has no type. Typing it, e.g. `len([]int(nil))` works fine.

> the len of an empty slice is 0, but the slice itself is == to nil

That’s not true. An empty slice is initialized: []T{} or make(T[]); it’s not equal to nil.[1] The zero value nil slice is technically not an “empty slice”. Colloquially you may call a nil slice an empty slice, but the nil-ness is still an important distinction that manifests in e.g. encoding/json.Marshal; nil marshals to null, whereas an initialized empty slice marshals to [].

If you want to test the emptiness of a slice, test the length, don’t compare it to nil.

[1] https://go.dev/play/p/IP2NIgwvaTR?v=gotip

Empty slices also serialize to `null` instead of `[]` when using the default json encoder.