Monday, 10 July 2017
go - Why does golang allow named slice type assignment without explicit type conversion?
Answer
Answer
I thought go didn't allow any named type to actual type assignment without explicit type conversion.
But how does it compile without an error if I assign []byte to json.RawMessage?
var a json.RawMessage // type RawMessage []byte
var b []byte
a = b
var x time.Duration // type Duration int64
var y int64
x = y // ERROR: cannot use y (type int64) as type time.Duration in assignment
https://play.golang.org/p/oD5LwJl7an
Answer
int64 is a named type, []byte is an unamed type.
Named types are specified by a (possibly qualified) type name; unnamed types are specified using a type literal, which composes a new type from existing types - golang spec
Also
Two named types are identical if their type names originate in the same TypeSpec. A named and an unnamed type are always different. Two unnamed types are identical if the corresponding type literals are identical, that is, if they have the same literal structure and corresponding components have identical types. - golang spec
Therefore
type MyByteArray []byte
type MyInt int
var a MyByteArray
var b []byte
a = b // legal because array is unnamed - their corresponding type literals are identical
var x MyInt
var y int
x = y // illegal because int is named - they don't originate in the same type spec
Also see Why can I type alias functions and use them without casting?
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