Question

I am trying to type assert from a []Node, to []Symbol. In my code, Symbol implements the Node interface.

Here is some surrounding code:

 43 func applyLambda(args []Node, env Env) Node {
 44     if len(args) > 2 {
 45         panic("invalid argument count")
 46     }
 47     fixed, rest := parseFormals(args.([]Symbol))
 48     return Func{
 49         Body: args[1],
 50         FixedVarNames: fixed,
 51         RestVarName: rest,
 52     }
 53 }

Here’s the error I get:

./builtins.go:47: invalid type assertion: args.([]Symbol) (non-interface type []Node on left)

I’m sure there’s a good reason for this. What’s the best way to proceed?

Answer 1

In saying x.(T) variable x should be of interface type, because only for variables of type interface dynamic type is not fixed. And while Node is an interface, []Node is not. A slice is a distinct, non-interface type. So it just doesn’t make sense to assume a slice of interface values is an interface too.

Type Node has a clear definition in your code and thus is an interface. You have specified the list of methods for it. Type []Node isn’t like that. What methods does it define?

I understand where your are coming from with this. It may be a useful shortcut, but just doesn’t make sense. It’s kind of like expecting syms.Method() to work when syms‘s type is []Symbol and Method is for Symbol.

Replacing line 47 with this code does what you want:

symbols := make([]Symbol, len(args))
for i, arg := range args { symbols[i] = arg.(Symbol) }
fixed, rest := parseFormals(symbols)