[Haskell-cafe] Type-level lazy bool operations

[Oleg]

Dmitry Olshansky wrote:
> There are some type-level boolean operation (If, &&, ||, Not) in
> Data.Type.Bool.  Are they lazy enough?
>
> I faced with problem and it seems that the reason is non-lazy "If".
> I.e. both (on-True and on-False) types are trying to be calculated.
> Does it work in this way really? Could it be changed?

Just a remark: if (type-level) computations are pure, then whether
they are done lazily or in any other way should not matter. The
evaluation strategy invariance is the main characteristic, if not the
definition, of purity.

Type-level computations are not pure however: they may have an effect
of divergence, or generating a type-error. Thus we really need a
type-level If, which evaluates only one of the two conditional
branches but never both. It can be easily implemented using the
standard approach of delaying evaluation with a thunk. Please see

        http://okmij.org/ftp/Haskell/TTypeable/TTypeable.hs

and search for ORELSE (near the end of the file). See
        http://okmij.org/ftp/Haskell/typeEQ.html#TTypeable
for background.