interface conversion: interface {} is nil, not string (1:9)

[mbardelmeijer]

When running the following expression, it returns in the error interface conversion: interface {} is nil, not string (1:9)

We would like to handle this gracefully, as the query parameter are optional that we're matching on. We prefer not to wrap it in string(), as our customer may create their own expression, and prefer to have this expression as clean as possible.

Is there any way to combat this, with for instance AllowUndefinedVariables? Or can expr-lang be modified to allow contains queries etc. on possible nil types?

env := map[string]interface{}{
    "query": map[string]interface{}{
        "another_query": "test",
    },
}

expression := "query.b contains 'test'"
prg, _ := expr.Compile(expression, expr.Env(env))
result, err := expr.Run(prg, env)
fmt.Println(result)
fmt.Println(err)

Output:

interface conversion: interface {} is nil, not string (1:9)
 | query.b contains 'test'

[antonmedv]

I think we should make it possible to compare to nil anything.

I will update == operator to support this.

[antonmedv]

Actually, the problem is not with ==, but with contains.

The == already supports this:

func TestExpr_string_nil_equals_string(t *testing.T) {
    var str *string = nil

    env := map[string]any{
        "nilString": str,
    }

    program, err := expr.Compile(`nilString == "hello, world"`, expr.Env(env))
    require.NoError(t, err) // Pass

    output, err := expr.Run(program, env)
    require.NoError(t, err)
    require.Equal(t, false, output) // Pass
}

[antonmedv]

Here is the test, only first is pasing:

func TestExpr_nil_op_str(t *testing.T) {
    // Let's test operators, which do `.(string)` in VM, also check for nil.

    var str *string = nil
    env := map[string]any{
        "nilString": str,
    }

    tests := []struct{ code string }{
        {`nilString == "str"`},             // ok
        {`nilString contains "str"`},
        {`nilString matches "str"`},
        {`nilString startsWith "str"`},
        {`nilString endsWith "str"`},
    }

    for _, tt := range tests {
        t.Run(tt.code, func(t *testing.T) {
            program, err := expr.Compile(tt.code)
            require.NoError(t, err)

            output, err := expr.Run(program, env)
            require.NoError(t, err)
            require.Equal(t, false, output)
        })
    }
}

[antonmedv]

Fixed.

[mbardelmeijer]

Awesome, thanks a lot! 🙏 -- Can confirm the fix works.