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

On Tue, Apr 14, 2009 at 9:24 AM, 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.

You have a point. I would like to avoid introducing unfounded authoritarian stigmas whenever possible. However, the way I see it is that unsafePerformIO *is* evil by itself, and it is only by the addition of Holy Water that it is benign to use. Ryan Ingram described it as a way to achieve "RTS extensions", which I think is a fine way to put it I consider Debug.Trace to be an instance of this: we are extending the RTS to provide execution traces. I guess it's a teaching style thing. Mostly, if someone sees "I have an IO [XmlTree] and I need an [XmlTree]", I want the "I'm asking the wrong question" synapse path to fire, rather than the "just use unsafePerformIO" one. Luke

More, from my personal experience, knowing unsafePerformIO helped me understand better what Monads are and how they should be used.

I wounder what so-called "experts" have to say about this.

Cristiano