It was recently brought to my attention that I had an error in a previous post. The error centered around setting a constant value of 5 to an interface. Here’s what I was doing.

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var x interface{} = 5
var x64 int64 = x.(int64)

The second line would result in the error panic: interface conversion: interface is int, not int64. The reason for this is that just the number 5 by itself is a constant in Go. Constants in Go, even though they may appear to be untyped, are implicitly assigned a default type based on their syntax. For example the constant 5.0 has an implicit type of float64 while 5 has an implicit type of int. Hopefully you can see what happened now.

When the code above runs, 5 is dumped into the interface variable x but the underlying type is int. Then when we try to assert that it’s int64, Go panics because there’s a type mismatch.

To fix the issue we need to tell Go what the type of the constant 5 should be with type conversion.

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var x interface{} = int64(5)
var x64 int64 = x.(int64)

Now everything runs as expected.