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

Stephen Tetley stephen.tetley at gmail.com
Sun Feb 14 18:09:28 EST 2010

Hi Evan

Singleton (aka wrap) would be nice - isn't it called Pointed in the
typeclassopedia but not otherwise existent? I suppose its missing by
historical accident rather than design.

I frequently use Semigroup (append but no zero) - there is one on
Hackage without any instances:

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.

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

Best wishes


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