Re: [Haskell-cafe] Type systems preventing laziness-related memory leaks?

Probably not. There's real code that depends on the current foldl semantics. On Wed, Feb 18, 2015 at 10:40 AM, Joe Hillenbrand wrote:

Is foldl = foldl' ever going to happen?

On Tue, Feb 17, 2015 at 11:50 PM, Roman Cheplyaka wrote:

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Note that foldr (+) 0 has nothing to do with laziness — it's eager. Its problem is that it's not tail-recursive.

foldl (+) 0 would be an example of a laziness issue.

If you want to specify which functions should or shouldn't be used in a lazy context, take a look at polarity (see Harper's Practical Foundations for Programming Languages). So you could say, for instance, that it doesn't make sense to use + in a lazy context; that'd eliminate half the cases of laziness-induced memory leaks in practice.

If instead you want to impose some upper bound on the memory consumption without caring about specific cases, then see this ICFP'12 paper: http://www.dcc.fc.up.pt/~pbv/AALazyExtended.pdf

On 18/02/15 07:04, Eugene Kirpichov wrote:

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Hello haskell-cafe,

Let me repost here a question I posted to cstheory stackexchange - in hopes that there are more type theory experts here.

http://cstheory.stackexchange.com/questions/29518/type-systems-preventing-la...

Perhaps the main source of performance problems in Haskell is when a program inadvertently builds up a thunk of unbounded depth - this causes both a memory leak and a potential stack overflow when evaluating. The classic example is defining sum = foldr (+) 0 in Haskell.

Are there any type systems which statically enforce lack of such thunks in a program using a lazy language?

Seems like this should be on the same order of difficulty as proving other static program properties using type system extensions, e.g. some flavors of thread safety or memory safety.

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