[Haskell] RFC: DData in hierarchical libraries

[Simon Peyton-Jones]

I'm glad to see this progressing.  However, it might be better to move
this thread to the libraries mailing list, which is specifically for
this purpose.   Anyone out there on the Haskell list who wants to
contribute to discussion about Haskell libraries?
	http://www.haskell.org/mailman/listinfo/libraries

Simon

| -----Original Message-----
| From: haskell-bounces at haskell.org [mailto:haskell-bounces at haskell.org]
On Behalf Of JP Bernardy
| Sent: 10 March 2004 23:27
| To: Christian Maeder
| Cc: Haskell Mailing List
| Subject: Re: [Haskell] RFC: DData in hierarchical libraries
| 
| 
| Hi,
| 
| > Maybe the documentation to the "0rdered lists"
| > section can be improved.
| 
| Could you be more specific about this?
| 
| > The functions Set.fromAscList and
| > Set.fromAscDistinctList should be
| > marked as "unsafe", since it's not clear what sets
| > result if the input
| > is not ascending (and/or has duplicates).
| 
| Would you prefix the function name with unsafe? I
| wonder what is the best way to do such a marking.
| 
| > Furthermore, already for
| > Set.fromList I expect it to be linear, if the input
| > list happens to be
| > ascending.
| 
| I'm afraid it's not, unfortunately. I intend not to
| fiddle with the implementation, to avoid the involved
| instability.
| 
| > "Set.map" is (still) missing. There are two variants
| > of map functions
| > with type:
| >
| > (Ord a, Ord b) => (a -> b) -> Set a -> Set b
| >
| > The first variant requires the function argument f
| > to be strongly
| > ascending, a < b => f a < f b, and corresponds to a
| > map on the
| > associated ascending lists.
| >
| > The second variant does not restrict the function
| > argument but may
| > result in a set of smaller size. Maybe this variant
| > should be called
| > "Set.image".
| >
| > The first variant, maybe call it "Set.mapAsc", is
| 
| I'd choose "mapMonotonic", from Edison.
| 
| > again "unsafe", since
| > the argument function may not be ascending. (The
| > descending case can be
| > ignored.)
| >
| > The same argument about "ordered lists" apply to the
| > Map module. In
| > addition, it would be nice if there were a variant
| > of "Map.keys" that
| > returns a set of keys, since the invariant that the
| > keys are ascending
| > and distinct may easily get lost, if not captured by
| > the Set type. A
| > straight forward implementation is:
| >
| > Map.keySet = Set.fromDistinctAscList . Map.keys
| >
| > The use of "Set.fromDistinctAscList" is safe in this
| > case, but - alas -
| > Map needs to import Set. (But maybe there is even a
| > faster
| > implementation of Map.keySet that exploits the
| > internal representation.)
| 
| Fine. I'll come up with a revision soon.
| 
| Thanks for your feedback,
| JP.
| 
| 
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