Comments on: The Point of Laziness

By: Ashley Yakeley

Ashley Yakeley — Wed, 27 Apr 2011 05:39:08 +0000

That’s pretty much the “social benefit” point that ee8a91jjf makes earlier, that SPJ himself claims.

It might be true, but I think one can still want an eager pure language.

By: Abstract Type

Abstract Type — Wed, 27 Apr 2011 04:23:03 +0000

Alan, I must’ve missed your point. I’m simply saying that there are no inductive types in Haskell, which I think there are not.

By: Abstract Type

Abstract Type — Wed, 27 Apr 2011 04:14:04 +0000

I plan to explain this remark in more detail in a later post.

By: sclv

sclv — Tue, 26 Apr 2011 19:01:28 +0000

In GHC Haskell (which is an extension of Haskell98, mind you), there do exist imprecise exceptions. However, denotationally, bottom is simply identified with the set of all possible exceptions. So you don’t have imprecise exceptions in addition to bottom — you still just have bottom. In genuine pure Haskell, these bottoms are all indistinguishable. It’s only in verybad-no-good-impure-anything-goes-IO-land that one can catch them. And even then, there’s a deliberate nondeterminism encoded in the semantics of catching exceptions. See “A semantics for imprecise exceptions” by Jones, Reid, Hoare, Marlow and Henderson for more.

By: summermute

summermute — Tue, 26 Apr 2011 18:32:21 +0000

Some comments on reddit were deleted, so the arguments can not be inferred from that discussion. Shortly, in a lazy language we have an equation
fst (x,y) = x (where (x,y) is of product type), which does not hold true in the case of a language with strict semantics (evaluation of ‘y’ may not terminate). Sum types have dual behaviour, as was illustrated here in other comments, but an example which gives a clear evidence of it seems to be trickier to construct.

By: tpnyberg

tpnyberg — Tue, 26 Apr 2011 13:09:04 +0000

In addition to bottom, doesn’t Haskell also include exception values for every type–ostensibly to allow pure code to “throw” exceptions?

By: will87

will87 — Tue, 26 Apr 2011 09:05:43 +0000

Well then, I rescind my earlier point and agree that, unlike ML, Haskell does not have natural numbers. Learnt a lot thanks :)

By: summermute

summermute — Tue, 26 Apr 2011 07:49:21 +0000

>I am puzzled by this remark There is a discussion on reddit (sorry) which tries to explain this moment.

By: Abstract Type

Abstract Type — Tue, 26 Apr 2011 05:57:11 +0000

If I remember correctly, it was in his MSc thesis from around 1990. I may well have learned this from him directly, however, I’m not sure.

By: roshanjames

roshanjames — Tue, 26 Apr 2011 05:31:25 +0000

I am puzzled by this remark: “As Andrzej Filinski pointed out decades ago, whereas lazy languages have products, but not sums, eager languages have sums, but not products.”

Could I know which of Filinski’s papers is being referred to here?