Saturday, September 12, 2009

More F# under the covers

This is a bit of a follow up to

and was provoked by running NDepend over -- naturally -- my current .net project as a first test drive (and being a work in progress, the debug build). Which then it highlit a number of quite simple methods involving function chaining as being candidates for refactoring (as well as the tut-tutting at the code generated for the Either< 'a,'b > type).

Let's look that series of short-circuitable tests, mentioned in the first post, something that would look like

in C#.

Let's have a type that expresses the intent more directly than a general purpose boolean, and a couple of helpful combinators...

Then we can write the chained tests as

Reflector shows us that the IL that comes out of the process in a debug build corresponds to code that looks like

If the Test function being used is a member function of the class, you get temporaries like

defined and being passed into the local lambdas; and there is no re-use of these temporaries.

Fortunately, the code generated in a release build avoids creating one-shot variables for these intermediate input values (and optimizes away the call to ignore) -- but it still has a separate tuple value for each test, rather than re-using a single name. Chain enough calls together and the number of hidden variables will start to trigger metrics-based alarms.

It looks like F# code is going to be rather heavy going for tools to work with.

This is unchanged with the Feb 2010 CTP (1.9.9.9).

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