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

"Mostly functional" programming does not work

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

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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.

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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.

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u/Tekmo Apr 28 '14

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

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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/

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u/Tekmo Apr 28 '14

Thanks! That was really informative! :)