I am having a hard time using a custom Error type in Go. I read this Blog post on Errors
So I tried this:
In my model.go I defined a custom error:
type ModelMissingError struct {
msg string // description of error
}
func (e *ModelMissingError) Error() string { return e.msg }
In one of my methods I throw a custom error like this:
...
return Model{}, &ModelMissingError{"no model found for id"}
...
In the caller of that method I would like to check the error returned for its type and take action if it is in fact a ModelMissingError
.
How can I do this?
I tried this:
if err == model.ModelMissingError
The result is *type model.ModelMissingError is not an expression*
Clearly I am missing something.
Reading the Blog post further exposes a bit of Go like this:
serr, ok := err.(*model.ModelMissingError)
This is the comma ok idiom, clearly I need to re do my go lang tour
A type assertion provides access to an interface value's underlying concrete value.
t := i.(T)
This statement asserts that the interface valuei
holds the concrete typeT
and assigns the underlyingT
value to the variablet
. Ifi
does not hold aT
, the statement will trigger a panic. To test whether an interface value holds a specific type, a type assertion can return two values: the underlying value and a boolean value that reports whether the assertion succeeded.