Re: [Haskell-cafe] Robert Harper on monads and laziness

I completely agree that laziness enables a number of nice coding idioms and, as Lennart described so eloquently, it does facilitate a combinator-based coding style among other things: http://augustss.blogspot.com/2011/05/more-points-for-lazy-evaluation-in.html (Note that even Bob admits that this is an advantage.) What I meant is that if asked what is more important about Haskell, its laziness or purity, I think most people would pick purity. (But then it's a strange decision to make as laziness implies a need for purity as discussed.) Manuel Jan-Willem Maessen:

On Tue, May 3, 2011 at 1:32 AM, Manuel M T Chakravarty wrote:

...

... Interestingly, today (at least the academic fraction of) the Haskell community appears to hold the purity of the language in higher regard than its laziness.

As someone who implemented Haskell with quite a bit less laziness, I'm inclined to agree.

That said, I think it's easy to underestimate just how much of the structure of the language really encourages a lazy evaluation strategy. One example: where blocks scope over groups of conditional RHSs. This is very handy, in that we can bind variables that are then used in some, but not all, of the disjuncts. Grabbing the first example that comes to hand from my own code:

tryOne (gg, uu) e | not (consistent uu) = (gg', uu) | uu==uu' = (gg, uu) | otherwise = (gg', uu') where gg' = gg `addEquation` e uu' = uu `addEquation` e

This kind of thing happens all over the place in Haskell code -- it's a very natural coding style -- but if you want to preserve purity it's tough to compile without laziness.

-Jan-Willem Maessen