\chapter{Conclusion}

In this thesis we have looked at ways to generically add position information
to recursive datatypes. We have looked at ways in which producers and
consumers can be written down in such a way that the position information is
constructed or consumed automatically.

The solution we have chosen is based on the fixed point view of datatypes. By
expressing a recursive type as a fixed point, it is possible to insert
position information at every recursive position.

We have implemented this solution in two ways: one using base functors
(Chapter \ref{sec:fixedpoint}), where the programmer decides beforehand that
the datatype is written with a type parameter for the children, and the other
in terms MultiRec (Chapter \ref{sec:generic}), a library for generic
programming for mutually recursive datatypes.

The former implementation provides good type-safety in all aspects: the shape
of the annotated datatype was exactly right, the producers can clearly
distinguish between annotated and unannotated trees, and the algebras that we
have developed can work with trees either with or without annotations. The
major drawback of this implementation is that it can only work with a single
datatype.

The latter implementation solves the drawback of the first one: the generic
programming library MultiRec, with which the second implementation is built,
is designed to work with mutually recursive datatypes. By mimicking the family
of datatypes with generic pattern functor building blocks, generic functions
and datatypes can be built. But this is also the main problem: the generic
constructors are not the same as the original ones, making it very difficult
for the producer (in our case the parser) to build recursively annotated trees
in an elegant way. Rather than ask the programmer to use the generic
constructors (comprised of compositions of |L| and |R|), we allowed the
original constructors for the price of a stateful construction model using
monad transformer |YieldT|.

With this, we have provided the first steps of turning generic position
information from a design pattern into a library.