On 12/16/2010 08:46 AM, Jun HU wrote:
My first language is C, and I've strong intention in learning a pure functional programming language. My very first question is how to think in the functional programming way, anyone has some ideas. Really appreciate....
I cannot claim C as my first language, but I do say it is my "natural" language; I have written more code in it than I have in any other, and I've been using it for nigh on 20 years. I also cannot claim to think in the functional programming way, but it definitely has changed my thinking. (Thinking about it now, I believe functional programming has had a bigger effect than I had considered. But I am definitely not a Haskell guru.) I believe the biggest difference between functional programming and procedural is thinking equationally rather than operationally. With procedural programming as in C (and object oriented languages, which in my experience are more procedural than not), the tendency is to think operationally: "First this happens, then it does this, then that,...." Now, back at UT Austin (I grew up in the land of Dijkstra), at the time anyway, they were fond of axiomatic thinking: "At this point in the program text, the state is this; at that point in the text, the state is that...." That is definitely a significant improvement, and I attribute what decent code I have written to that approach, but it definitely is not as natural as either operational or equational thinking. In Haskell especially, but in other functional languages to a lesser extent (and also when doing functional approaches in a non-functional language), it is very important to think equationally: "This means that; this other thing is that...." Now, equational thinking is easy for stuff like add_one n = n + 1 But it is not immediately apparent how that extends to something that needs recursion, say. The big "Ah-ha!" moment for me is the realization that recursion and mathematical induction are the same thing (ok, it's not a surprise; the big deal was when I internalized it). With that, you can deal with anything equational and recursive as equational and inductive. The change in thinking is as big as the difference between operational and axiomatic. Equational thinking extends beyond basic function definitions; I think it is the key to the type system, classes, and most of the other neat language features. At this point, I am still struggling with category theory, although I have been able to make use of monads reasonably elegantly (in my (humble) opinion, of course). I think there is another step there, although I haven't made it. (I am willing to change my traditional definition, "Programming is applied formal logic," to include at least abstract algebra and category theory at this point, though.) -- Tommy M. McGuire mcguire@crsr.net