Base including notnan, notnothing, ...?

Internals & Design
3

Yes, to see:

julia> !isnan
#84 (generic function with 1 method)

so writing !isnan returns a function already. This is documented behaviour of !:

help?> !

(...)

  !f::Function

  Predicate function negation: when the argument of ! is a function, it returns a function which computes the boolean negation of f.
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