Henning Thielemann schrieb:
in haskell-prime list there was a proposal to use '?' for such things:
(1,2,?)
only problem is what it's hard to define exactly where the lambda should arise:
the placeholder '?' becomes part of a new implicit identifier (that exceeds Haskell's identifier syntax). If we have: (,,) :: a -> b -> c -> (a, b, c) we get a bunch of new identifiers: (?,,) :: b -> c -> a -> (a, b, c) (,?,) :: a -> c -> b -> (a, b, c) (,,?) :: a -> b -> c -> (a, b, c) And maybe also: (?,?,) :: c -> a -> b -> (a, b, c) and so on. For infix ops this looks natural to me.
(1,2,\x->x)
I see no reason for this interpretation.
(\x -> (1,2,x))
That should exactly be the implicit definition of my above implicit identifiers.
Yes that's a really evil problem that I already encountered in mathematics. Some mathematicians like to write f(·) or even f(·-k) which exhibits exactly the ambiguity you mention. Such placeholders are a really bad idea.
"f(?)" should not make sense in Haskell, since the parens do not belong to the identifier "f". (It's also not necessary: \ x -> f(x) = f) Cheers Christian