On Wed, 25 Sep 2002 16:06:29 -0700 (PDT)
Hal Daume III
I don't feel bad about doing this because GHC does this itself for its own configuration :).
I am going to show you that using unsafePerformIO where there really are side effects leads to unpredictable results, and is generally wrong in a lazy language. Don't hate me for this :) Consider this example (supposing that a Config is represented by an Int): storeConfig :: Int -> () readConfig :: Int They both are obtained through the use of "unsafePerformIO". Now, say I got this code: (storeConfig 0,storeConfig 1,readConfig,storeConfig 0,readConfig) What is this 5-uple supposed to evaluate to? First of all, this depends on order of evaluation. We can't say that all the elements of the tuple will be evaluated, so we can't tell if the fifth readConfig will evaluate to 0 or 1 (if the third storeConfig is never evaluated, readConfig will evaluate to 0, else to 1) This is one of the causes of the use of monads: ensuring correct order of evaluation. Second, suppose we were able to force order of evaluation (which shouldn't be allowed, in a lazy language). We still can't say what the last "readConfig" would evaluate to, since we don't know if the compiler is substituting equals for equals (I am expecting a lazy functional language to do this). If the compiler does, the last readConfig is equal to the first (in fact, by the use of unsafePerformIO, you have told the compiler that both the functions storeConfig and readConfig are pure, which is not true) and will evaluate to 1, else it will evaluate to 0. And, besides, the compiler should also substitute the second "storeConfig 0" with the result of the first occurrence, so it would not evaluate the second "storeConfig" at all. This is another example of the need for monads: allowing program transformations, first of all substituting equals for equals. This is why (even if, by enough knoweledge of the implementation, we could), by only relying on the semantics of a lazy language, we can not have functions with side effects. If it wasn't so, they would not have invented monads, believe me. I apologize, as always, for my terrible english, and hope I have been clear. Vincenzo Ciancia