5.20. LazyST

This library is identical to ST except that the ST monad instance is lazy. The lazy ST monad tends to be more prone to space leaks than the strict version, so most programmers will use the former unless laziness is explicitly required. LazyST provides two additional operations:

lazyToStrictST :: LazyST.ST s a -> ST.ST s a
strictToLazyST :: ST.ST s a -> LazyST.ST s a 

These are used to convert between lazy and strict state threads. The semantics with respect to laziness are as you would expect: the strict state thread passed to strictToLazyST is not performed until the result of the lazy state thread it returns is demanded.