specify call-by-need

John Meacham john at repetae.net
Wed Feb 16 03:12:27 CET 2011

Except for the fact that compilers don't actually implement call by
need. An example would be the speculative evaluation of ghc.


And local optimizations that affect asymptotic behavior are used all
the time, to the point they are vital for a functioning compiler. The
tail-call optimization turning O(n) space usage to O(1) being a prime

And what is meant by "call-by-need" in the presence of exceptions and
concurrency is not entirely obvious.

I think that specifying call-by-need would be more confusing and
contrary to what actually exists in the wild.


On Tue, Feb 15, 2011 at 5:53 PM, Scott Turner <2haskell at pkturner.org> wrote:
> In practice, Haskell a call-by-need language.  Still, software
> developers are not on firm ground when they run into trouble with
> evaluation order, because the language definition leaves this open. Is
> this an underspecification that should be fixed?
>  1. Haskell programmers learn the pitfalls of sharing as soon
>     as they cut their teeth on 'fib',
>  2. Virtually all significant-sized Haskell programs rely on
>     lazy evaluation and have never been tested with another
>     evaluation strategy,
>  3. Questions come up on Haskell-Café, infrequently but regularly,
>     regarding whether a compiler optimization has altered sharing
>     of values within a program, causing it to fail,
>  4. The rationale for the monomorphism restriction assumes
>     lazy evaluation,
>  5. It is the effect on asymptotic behavior that matters,
>  6. Portable Haskell code should not have to allow for the
>     variety of non-strict evaluation strategies, as the Haskell
>     Report currently implies.
> I suggest specifying call-by-need evaluation, allowing for the places
> where type classes prevent this.  If necessary, make it clear that local
> optimizations not affecting asymptotic behavior are permitted.
> This would not eliminate struggles with evaluation order. The intent
> would be to clarify expectations.
> -- Scott Turner
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