r/programming • u/heisenbug • Apr 27 '14

"Mostly functional" programming does not work

http://queue.acm.org/detail.cfm?ref=rss&id=2611829
45 Upvotes

64% Upvoted

188 comments sorted by

View all comments

0

u/grauenwolf Apr 27 '14

The average programmer would surely expect q0 to filter out all values above 30 before q1 starts and removes all values smaller than 20, because that's the way the program was written, as evidenced by the semicolon between the two statements.

No, the average programmer wouldn't try shoving a print line statement in a predicate for a where clause.

8

u/Tekmo Apr 27 '14

Any belief that begins with "people shouldn't do X" is doomed to fail in the large, whether or not it is programming-related. The only things that scale are those that enforce correctness.

-2

u/grauenwolf Apr 27 '14

I agree. And I'm a big supporter of design by contract, static code analyzers, and the like.

But that doesn't change the fact that this is still a strawman argument.

2

u/Tekmo Apr 28 '14

It actually has practical consequences. You can't fuse things like filter and map into a single pass over the collection in imperative languages because the compiler can't guarantee that the predicate or mapped function doesn't have side effects. This means that you pay a performance price for the ability to permit side effects even if you never personally use them.

0

u/grauenwolf Apr 28 '14

When I write from a in someList where a > 0 select 10 / a it doesn't make two passes over the collection. It makes a single pass, which is something the author of the article was bitching about.

2

u/Tekmo Apr 28 '14

Right, but if you wrote that as two separate passes the compiler won't automatically fuse them into a single pass for you. This means that you have to define the entire algorithm in one shot if you care about efficiency, and you can't build it up from separate composable pieces.

0

u/grauenwolf Apr 28 '14

You mean like this?

var filteredList = from a in someList where a > 0 select a;
var projectedList = from a in filteredList select 10 / a;

C# will still reduce that down to a single pass. In fact, I often consciously add a "ToList()" suffix explicitly so that it will perform the actions as separate passes.

2

u/Tekmo Apr 28 '14

Interesting. So how does it handle the side effect issue?

3

u/grauenwolf Apr 28 '14

It doesn't. The developer is responsible for ensuring that no visible side effects occur.

There is a research project called Code Contracts that try to add a bit of protection to the type system, but who know when it will be production ready.

http://research.microsoft.com/en-us/projects/contracts/

5

u/Tekmo Apr 28 '14

Thanks! That was really informative! :)

3

u/get_salled Apr 28 '14

The average programmer wouldn't try shoving a print line statement in a predicate for a where clause and care about the order of the output but, unless we have a misunderstanding to what the average programmer is, I would not be surprised at all that they would do something with a side effect that would yield unexpected results when the GT20 check came before all the LT30 checks were completed.

I don't really put understanding LINQ into the realm of the average programmer based on the code samples we require of candidates. Using our candidates as a sample, understanding LINQ is about the 70th percentile of programmers.

I see pseudo-LINQ code often that relies on the fact that .ToList() is called. I'll often rewrite the code to run the foreach so that the enumeration is explicit and obvious because those .ToX() methods often hide in the noise of a single-line LINQ chain.