I think there is another problem with having strict constructors. It messes up parametricity even more than fix does. There are two reasons why this would be a shame: * Parametricity is cool. It lets you prove lots of interesting theorems for very little work. These theorems help with program transformation and the like. * Some compilers use parametricity. In particular, the justification for cheap deforestation method (foldr-build) comes from parametricity. If parametricity is weakened too much the transformation may become unsafe. One way to introduce strictness is to use overloading and have a class Strict with an operation strict : a -> a defined for each type in the class (not including functions unless their semantics changes, nor unlifted products if they get introduced). Then a strict constructor would have a class restriction and these would provide the standard mediation for parametricity. John.
