[Haskell-cafe] What is the meaning of tilde ("~") symbol

Evan Laforge qdunkan at gmail.com
Sun Feb 14 18:41:06 EST 2010

> I'm no fan of (!!) on lists or other containers where it isn't O(1),
> but lookup/member are a bit more promising. However are there any
> useful derived operations or constructions that can be defined only in
> terms of a Lookup type class? For comparison, Monoid has mconcat as a
> derived op and e.g. the Writer monad can be usefully abstract by
> relying only on the Monoid interface as can Foldable.

I was thinking of (!!) as an unsafe lookup, where lookup is the safe
one.  Of course historically (!!) considers lists as (Int -> elt)
while elem considers them a set, which is why you'd need some newtype
wrappers.  Anyway, I'd be happy to not include the unsafe variants at
all.  That would mean putting [] into Setlike with 'member' but not
Maplike with 'lookup'.  You'd have to come up with a better name or
hide the Prelude alist lookup.

I don't know about derived operations, I suppose you could come up
with some sort of Evironment monad that required merely a (Maplike a)
class constraint instead of a concrete data type like Map, but for me
the prime motivation would be the short universal name.  I could stick
it in my "import unqualified" toolbox and type less.

> Excepting Data.Tree, Data.Graph and HashTable, 'containers' seems
> pretty regular now - it certainly has moved on with

Yes, and it's not a big deal to me.  I use Map and Set and List and
the overhead of separate functions for each is pretty minimal.  If I
used a wider variety of containers I'd probably want it more.

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