On Fri, Oct 17, 2008 at 07:58:45PM +1300, Jamie McCloskey wrote:
All this creates a whole load of functions with IO, even though it should not be necessary. What I would like to know, is how I can avoid this, possibly by modifying exec to just take a State.
The short answer is: it IS necessary. Think about what exec does, for instance. It takes a Program and an initial State and does... what? Executes the program, which might result in some things getting printed to the screen or some input being read from the keyboard. That is, exec can have some I/O effects. Therefore, exec's return type must be an IO value. The same goes for all the other functions you mentioned. This is a feature, not a bug --- the type of a function tells you precisely whether it can possibly have any I/O effects. If the return type is (IO something), then it can; if the return type does not involve IO, then it is *guaranteed*[1] that it cannot have any I/O effects. -Brent [1] Except for that pesky unsafePerformIO. But you should never use that, unless you really absolutely know what you are doing and why.