[Haskell-cafe] A Question of Restriction

[Wouter Swierstra]

Hi Brian,

If I understand you correctly, you've run into the "Expression  
Problem". Phil Wadler posed the problem in a widely-cited e-mail,  
formulating it much more clearly than I ever could:

   http://homepages.inf.ed.ac.uk/wadler/papers/expression/expression.txt

There are lots of ways to tackle this problem in Haskell - just google  
"Expression Problem Haskell". My (completely biased) personal  
favourite is:

   http://www.cse.chalmers.se/~wouter/Publications/DataTypesALaCarte.pdf

Hope this helps,

   Wouter