As an esoteric programming language, Unlambda is meant as a demonstration of very pure functional programming rather than for practical use. Its main feature is the lack of conventional operators and data types -- the only kind of data in the program is one-parameter functions. Data can nonetheless be represented with curried functions. Unlambda is based around the principal of abstract elimination, or the elimination of all saved variables, including functions. As a purely-function language, Unlambda's functions are not only first-class objects, but they're the only first-class objects. Unlambda also has no side-effects whatsoever.
The hello, world program in Unlambda is somewhat odd. It is below:
`r```````````.H.e.l.l.o. .w.o.r.l.di