In Golang, zero values are used in case of an error. Eg, if a key is absent in a map[int]string, an empty string is returned.
I'm writing a function that takes in a url and returns the json string received on making an http request. If the error is not null, I would like to set the return value for the body to be a zero value byte array. Since a byte array can be cast into other data types, I can't figure out what to set it as. Maybe context dependent? Since I expect the returned value to be a string, I should set it to an empty string?
No, zero values are not idiomatically used as in-band error values. Map returns a zero value for keys not present, but that's an (often used) feature as one can, if required, test for the presence by the alternative syntax
v, ok := m[k]
Use multiple return values to return a value and a signal at the same time.
To signal a simple (true/false, ok/not ok, ...) condition, use a boolean value.
To signal an error condition, use an error typed value.
Etc.
BTW: The Go programming language has no casts. It has conversions and they're not the same as casts.