Because sometimes this is a win, but sometimes the overhead of forking off a separate thread and synchronising on the result costs more than the benefit. Compil

Because sometimes this is a win, but sometimes the overhead of forking off a separate thread and synchronising on the result costs more than the benefit. Compilers are very bad at figuring out which is which. Haskell lets you add parallelisation instructions to tell the compiler which is which without changing the semantics of the program.