[Haskell-cafe] Ord methods too strict?

Li-yao Xia lysxia at gmail.com
Wed Jan 2 15:06:57 UTC 2019

Hi Vilem,

 > This is not just about crashing. (I’m using `undefined` as a way of
 > making strictness explicit.) `False >= veryExpensiveComputation`
 > should return `True` immediately without any unnecessary computation.

There is also an opposite situation, where evaluating the second 
argument allows us to free a lot of memory, as that argument would 
otherwise remain a thunk with references to many things.

 > Also it doesn’t seem like a special case: this makes sense for any 
partially ordered Type with a top and/or bottom element.

That may be acceptable with Bool because its values are small. But to 
compare two pairs `(a,b) <= (x,y)` we would have to first traverse 
`(a,b)` to decide whether it's the smallest element, before evaluating 
the second argument to WHNF and comparing component-wise. Besides the 
extra complexity (both in code and in run time), this would not be less 
strict than the current default, since the first argument could get 
completely evaluated before even looking at the second one: `(False, 
_|_) <= (True, True) = _|_` (currently, this is `True`). We also would 
not know whether the type (a, b) has a smallest element without stronger 
constraints than (Ord a, Ord b), so this optimization is really only 
practically feasible for types with a nullary first constructor.


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