On Thu, Mar 17, 2016 at 6:47 PM, Richard A. O'Keefe
Let me rephrase that. To me, that "if it does actually get to c" bit is NOT a consequence of my understanding of the general rules for enumeration in Haskell, they are a complicating ADDITION to those rules just for a case that I would be very upset to see happening in code of mine.
Indeed! I agree completely that this is not consistent with the current rules in Haskell, and I agree completely that this is a complicating addition to those rules that is in no way derived from the way things currently work or from some clear greater principle.
I mean, this is an *extremely* special case. [1.0,1.0..x] :: [Double is an infinite list for ALL x >= 1.0 in Haskell; you want to change this to be [1.0] if x happens to be the very special case 1.0, and I do not understand why. Why is [1.0,1.0..1.0+epsilon] being infinite, [1.0,1.0,..1.0-epsilon] being empty, but [1.0,1.0..1.0] having one element USEFUL? (And while you are at it, explain your reasoning for [x,x..negate x] when x is 0.0.)
To be clear: I’m not proposing this, I’m just exploring how such a rule may be specified. Although my current intuition agrees with the original post’s stated intuition in that I was surprised to find [1,1..1] is not finite, I personally don’t think it’s necessarily better to have an intuitive rule: I generally favor rules that are simple to state and reason about, rather than rules that produce superficially intuitive results at the expense of special cases and conceptual complexity — and I certainly hope saying that does not derail this conversation into other recent discussions with similar tradeoffs. ;-) If I were to propose this, which I’m not, I would discuss what the [a,b..c] notation is meant to represent. To me, personally and for no good reason, it looks like an iterator that yields numbers starting at a, adding (b-a) at each step and yielding the next number as long as it’s less than or equal to c, and continuing as long as c is neither reached nor exceeded. I make no claim on the universality of this interpretation or the naturality of its deduction as a result of some prior notational intuition, and I likewise make no claim about the adequacy of this intuition as the foundation for the rules in Haskell. In any case, I would personally never find it reasonable for a proposal to suggest changing the rules Haskell uses to assign meaning to this notation, unless there was very wide community consensus and interest in such a change, which I find doubtful as it’s such a minor detail. I would prefer to see this turn into, say, a very short note in the documentation pointing out this curiosity that may be inconsistent with the intuitive expectations of some of us.
[6,6..6] would have the initial 6, and then it should have no other element beyond 6, so it should in fact equal [6] under this intuition.
But [6,6,6,6,6,6,6,6,6,6,6....] ALSO has no other element beyond 6. "Not going beyond 6" is one thing, "stopping exactly at 6" is another.
I misspoke again, indeed! I should have said «it should have no other element beyond the first 6». Note I also purposely avoided cases with b < a and c < a, as those confuse me even further, and I have no opinion or solid intuition about them.