The dreaded M-R

Ben Rudiak-Gould Benjamin.Rudiak-Gould at
Tue Jan 31 14:47:47 EST 2006

Josef Svenningsson wrote:
> There are many evaluation strategies one can use to implement Haskell.
> The problem with the M-R is that it is a concern only in *some* of these 
> evaluation strategies, most notably lazy evaluation.

True, but it's a concern in any evaluation strategy that tries to avoid
multiple evaluation of let-bound expressions, which includes lazy,
optimistic, and eager evaluation. A strict dialect of ML with type classes
would face the same problems.

> If you read the motivation section which defines the M-R [...] the report
> suddenly starts to talk about how many times a certain a certain 
> expression is evaluated. But nowhere in the report is it defined how 
> expressions should be evaluated. This makes the M-R truly butt-ugly!

I agree, but you don't have to specify lazy evaluation in order to justify
the M-R. Some sort of nondeterministic graph reduction semantics would be
good enough.

-- Ben

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