presto-spi/src/main/java/io/prestosql/spi/type/TypeSignatureParameter.java/TypeSignatureParameter/jsonValue
class TypeSignatureParameter:    public String jsonValue()
    {
        String prefix = "";
        if (kind == ParameterKind.VARIABLE) {
            prefix = "@";
        }

        String valueJson;
        if (value instanceof TypeSignature) {
            TypeSignature typeSignature = (TypeSignature) value;
            valueJson = typeSignature.jsonValue();
        }
        else {
            valueJson = value.toString();
        }
        return prefix + valueJson;
    }