Dynamic Type Check

Recently some of our developers ran into the accusation that with a dynamic language like ruby you use so many dynamic type checks that you end up effectively writing your own type system. So they thought, since we've written a lot of real ruby code - how often do we make dynamic type checks? Michael Schubert gathered up the data.

The table below contains the data. We define a dynamic type check as the use of the methods is_a?, kind_of?, and instance_of?. The lines of code come from the standard rake stats command in rails.

Project ID	Code type checks	Code LOC	Test type checks	Test LOC	LOC / type check	test LOC / code LOC
A	16	13318	0	9856	1448	0.7
B	14	19138	0	17123	2590	0.9
C	0	2607	0	2981	∞	1.1
D	7	4265	3	4069	833	1.0
E	32	29619	60	97688	1384	3.3
F	18	~9500	N/A	N/A	528	N/A
G	0	2455	0	3290	∞	1.3
H	9	2220	6	6404	575	2.9
I	23	10633	2	12331	919	1.2
J	196	40461	24	88511	586	2.2
K	17	5769	6	9848	679	1.7

The moral of this data is that you shouldn't expect to see a lot of type check calls in your ruby code base. This, of course, is true of any dynamic language. It was generally considered bad form in Smalltalk circles I inhabited too.

The methods that were checked for in this data aren't the only ones that can be considered a dynamic type check. Other cases are respond_to? and aClass === anInstance. Our folks felt that these cases were no more common than the ones they checked for.

Most uses are those of dealing with liberal input - eg where a method parameter can be a string, symbol, or array. These crop up in DSLish situations where you want liberal input for the high readability.