Why aren't computables used for numerical calculations?

Question

In programming languages like Haskell you can use lazy evaluation to delay calculations.

Why isn't a similar approach being used for numerical methods (I understand that there would be memory implications).

The approach would be that non-integer values would be stored as how they are calculated, so that the next digit can be found when requested.

This kind of approach should allow every value to be calculated to the correct number of significant figures.

Sorry that this is slightly garbled.