  Property-based testing is a powerful method to validate program correctness.
It is, however, not widely use in industry as the barrier of entry can be very
high. One of the hindrances is to write the generators that are needed to
generate randomised input data. Program properties often take complicated data
structures as inputs and, it requires a significant amount of effort to write
generators for such structures in a invariant preserving way.
  In this paper, we suggest and formalise a new programming language
\textsf{pun}; a simple functional programming with properties as a built-in
mechanism for program validation. We show how to generate input for
\textsf{pun} properties automatically, thus, providing the programmer with a
low barrier of entry for using property-based testing. We evaluate our work a
on library for binary search trees and compare the test results to a similar
library in Haskell.
