The second exercise is to write the get function, retrieving an element
from a heterogeneous list.
In the glambda repo, look in the exercises directory and open up
Ex2.hs. The code we've been discussing should appear.
Write a function get :: HList tys -> Elem tys ty -> ty. A correct
definition of this function will tickle GHC's bug #3927 (fixed for GHC 8),
when you get a spurious warning about incomplete patterns.
If you have extra time left over at the end, start playing with glambda.
In the top level of the glambda repo, say
> cabal configure
> cabal build
> ./dist/build/glam/glam
A brief manual for the Glamorous Glambda interpreter is toward the bottom
of the README for this repo. You'll find some test .glam
files in the tests directory. Load them with :load tests/revapp.glam
for instance. (No quotes around filenames, please.)
The solution is available in Ex2.hs in the solutions branch.