From: Martin Vlk [mailto:vlcak01@tiscali.cz]
http://www-i2.informatik.rwth-aachen.de/Staff/Current/michaelw/sttt-ml-haske ll.pdf This quote from the paper resonated with me: "Also, if imperative elements of a given application were not taken into account during its design but turn out to be necessary later on, often major parts have to be redesigned or (at least) reimplemented, especially because types change significantly. A simple but recurring example is to add printing of status information to an otherwise purely functional algorithm. In the worst case this could result in having to rewrite the algorithm in a monadic style, but also to rewrite its callers (and transitively their callers as well), plus adjusting all type annotations on the way. Even when using opaque accessors to data structures, the required changes cannot necessarily be limited to a single module, but affect large parts of the system." The difficulty seems to be when you want to turn code that was initially "pure" into monadic code: it requires a fairly substantial rewrite. Once in a monadic style, I expect it is much easier to add various monadic/imperative enhancements. I've experienced this recently, where I've converted an algorithm from a purely functional version, using immutable arrays, to a monadic version using destructive arrays. I introduced errors with the conversion, and the unit test suite that I had for the pure version also had to be converted to a monadic style, in order to test the now-monadic code. So having to perform this conversion is clearly undesirable. I'm reminded of Wadler's "Monads for functional programming" (http://homepages.inf.ed.ac.uk/wadler/papers/marktoberdorf/marktoberdorf.pdf ) where he illustrates how easy it is to extend a program written in a mondadic style. So this begs the question: how much should we stick to a purely functional style? Should we advocate the adoption of a more monadic style from the outset, for programmers new to Haskell too? Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************