Re: Proposal: making inits and tails less strict

On 03/18/11 04:06, Bas van Dijk wrote:

On 17 March 2011 21:09, Don Stewart wrote:

...

Could the strictness properties be stated in the specification (i.e. include some QuickCheck properties that define the strictness)

Good point. I will add the following strictness properties to the docs:

inits ⊥ = [] : ⊥ tails ⊥ = ⊥ : ⊥

I'm not sure if we should add QC properties to the docs but we can have:

prop_lazyInits xs = head (inits xs) == [] ...

By the way, QuickCheck can't really check laziness properties. It always provides finite, non-bottom input. (Well unless you make a generator that is somewhat out of its usual mold.) I think there's a library like "StrictCheck" that is meant to check strictness/laziness properties? For the Report, I sort of like the idea of just writing things like you said

inits ⊥ = [] : ⊥ tails ⊥ = ⊥ : ⊥

Although consider, it's been interpreted that the Report already has binding strictness requirements (strictness must be equivalent to the Report's implementation). We might want to change that means-of-specification if some other means is better. I think that's a good, unambiguous means though, unless we *want* the Report to underspecify strictness. Other thought about the proposal: Often, changing strictness has unintuitive effects on performance (I don't remember if that came up for this particular proposal sometime in the past, or for some different list function that libraries@ had been wishing were slightly lazier).