The Glasgow Haskell Compiler uses tagged pointers to implement lazy evaluation. Suspended computations are created as heap-allocated "thunks", and the runtime w

The Glasgow Haskell Compiler uses tagged pointers to implement lazy evaluation. Suspended computations are created as heap-allocated "thunks", and the runtime will tag any pointers to a thunk that is known to have already been evaluated, which can save one indirection when you need to "force" the thunk to get a value. Furthermore, for thunks of algebraic data types with only a few alternatives (e.g. booleans, the Maybe type) the tag will encode which of the alternatives the pointer actually points to. So if you are case scrutinising a boolean you can decide which branch to jump to by inspecting the pointer only with no memory access. This last trick doesn't work if the number of alternatives is > 3 (for 2 byte alignment) because there aren't enough bits available.