unlambda package contains an interpreter written in Haskell for the Unlambda language. It is now available in Fedora. Install it using:

 $ sudo yum install unlambda

Unlambda is a “nearly pure” functional programming language. There are no named functions in Unlambda. The s and k primitive functions are part of the core language, and are sufficient to make Unlambda Turing complete. The backquote(`) is used for function application. There are no variables in Unlambda. r prints a newline, and i is the identity function.

 $ unlambda
`ri

 $

The k combinator takes two arguments (by currying), say X and Y, and is expressed as ``kXY, which evaluates to X (Y is also evaluated). The s combinator takes three arguments and is applied as ``FXYZ, which evaluates to ``XZ`YZ. The .x functions prints the character x to the output. So, the r function is an instance of .x function where x represents the newline character.

 $ unlambda
```k.Hii

H $

The classic hello world program:

$ unlambda
`r```````````.H.e.l.l.o. .w.o.r.l.di
Hello world

The following hello.unl program prints “Hello world!” followed by asterisk symbol, in incremental fashion.

```s``sii`ki
 ``s``s`ks	
     ``s``s`ks``s`k`s`kr
               ``s`k`si``s`k`s`k
                               `d````````````.H.e.l.l.o.,. .w.o.r.l.d.!
                        k
	k
  `k``s``s`ksk`k.*

$ unlambda < hello.unl

Hello, world!
Hello, world!*
Hello, world!**
Hello, world!***
Hello, world!****
Hello, world!*****
Hello, world!******
Hello, world!*******
Hello, world!********
Hello, world!*********
Hello, world!**********
...