Disappointment
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Date:  20080722 (12:58) 
From:  Nicolas Pouillard <nicolas.pouillard@g...> 
Subject:  Re: [Camllist] Disappointment 
Excerpts from Paolo Donadeo's message of Mon Jul 21 23:28:36 +0200 2008: > I'm disappointed with myself and my incredibly low IQ. Late this > evening I decided  and this is the third time  to be enlightened > by the concept of monad. > > I like functional programming, but monads [1] must be too little to be > grabbed by my mind. This time the interest in monads was aroused by > the interesting article of David Teller, Arnaud Spiwack and Till > Varoquaux [2] about the error monad, but for using the library they > wrote I need at least some knowledge about monads and the donotation. > > I tried with some tutorials found around, but I still cannot catch the > point: what the hell is a monad? Two key points that helped me: * Monads help to separate some plumbing from your code. * Monads provide a way to abstract code over some "let" construct. I will note that specific "let" construct "let!", it's somewhat like the donotation but more atomic. Monads also come with "return", but that's not the essence of them. Think about that example: val div : int > int > int option val square : int > option let f x y = match div x y with  None > None (* here 'y' was equal to 0 *)  Some z > match square z with  None > None  Some x2 > Some x2 In the previous example the plumbing is error handling (where errors are represented by None), and the "let!" construct is: let! x = e1 in e2 ===> match e1 with None > None  Some x > e2 And "return" is "Some". Another similar example: type ('a,'b) either = Left of 'a  Right of 'b val div : int > int > (string, int) either val square : int > (string, int) either let f x y = match div x y with  Left error_msg > Left error_msg  Right z > match square z with  Left error_msg > Left error_msg  Right x2 > Right x2 Here the plumbing is still there for "error handling", but is somewhat different. The "let!" construct is: let! x = e1 in e2 ===> match e1 with Left m > Left m  Right x > e2 And "return" is "Right". Finally one could have used the "let!" construct abstractly (and also "return"). let f x y = let! z = div x y in let! x2 = square z in return x2 One can obtain the two previous versions by choosing which "let!"/"return" we want, namely choosing the monad. Hope that helps,  Nicolas Pouillard aka Ertai