Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]

Cristiano Paris wrote:

On Tue, Apr 14, 2009 at 5:09 PM, Luke Palmer wrote:

...

... Please don't say that. He's a beginner. You realize that the path of least resistance will be to use it, right? You see why that's not a good thing? Even experts don't use this function. (To the O.P.: don't use it)

Mmmh, sorry Luke but I don't understand this ostracism.

unsafePerformIO is not "evil" by itself, it's there for a purpose and, as for anything else in the language, it's better to understand when to use it and when not rather than just knowing that is something that MUST not be used, without any further explanation.

Sure, the explanation is there if people are interested in it. However, in context, your answer was wrong. It is like someone asking: "How do I get hold of a new phone" and the answer "Pull a gun on someone walking down the street and demand they give you their phone" ...that is, the answer was solving the wrong problem, or solving it in the wrong context. If you have IO [XmlTree], then you don't have an [XmlTree] at all - rather you have a description of an (IO-involving) action which you need to run to get one. You can run it many times, or once, or never. It will (in general) give different results depending exactly when you run it. Therefore you need to carefully decide when to run it - i.e. attach it indirectly or directly into your main action, as the various other answers have shown. unsafePerformIO is not part of the haskell language - it does not respect the type system. It is an extension mechanism which allows us to add hooks into the RTS; effectively a way to extend the language. This is a useful and powerful thing, but nothing in the questioner's question suggested that language extension was what they wanted. Jules