why if we append data in slice and not use it (variable mySlice in the program), why not throw the error message "unused variable" golang. Thankyou
#example
var mySlice []string
mySlice = append(mySlice,"halo")
and same behaviour with map
#example
var my = map[string]int{}
my["tes"]=1
Implementation restriction: A compiler may make it illegal to declare a variable inside a function body if the variable is never used.
As you can see, a compiler may or may not accept unused variables. The current Go compiler implements this by checking if the variable is read. Reading a variable means the variable is used.
The "unused variable" (or rather "declared but not used") is given by the compiler if you don't read a variable.
For example:
var i int
i = 3
Even though you are assigning a value to i in the above example, since it is not read, this is a compile-time error.
Appending to a slice using append() involves reading the slice (the slice is passed to append()).
Assigning a value to a map key involves reading the map value.
Same goes if you don't use append() but "directly" assign a value to a slice element:
var i = []int{0}
i[0] = 1
This is OK because assigning to i[0] involves reading the slice header to determine the (address of the) element to which we're assigning.
But if we were to assign to i itself like this:
var i = []int{0}
i = []int{1}
This assignment doesn't involve reading i so it is also a compile-time error.
If we were to use a struct for example:
type Point struct{ x, y int }
var p Point
p.x = 1
This still compiles, even though the assignment doesn't really involve reading the p struct. We might think of it as "identifying" the p.x field involves accessing the p variable.
This is a little bit of "gray area" and is based on opinion of the Go authors, but this is how they decided and implemented it.