<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On 5 May 2009, at 11:27, z_axis wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="font-size: 10pt; margin-top: 10px; margin-right: 10px; margin-bottom: 10px; margin-left: 10px; font-family: verdana; "><div><font face="Verdana" size="2"><div>The following code snippets is from xmonad:</div><div></div><div>-- Given a window, find the screen it is located on, and compute</div><div>-- the geometry of that window wrt. that screen.</div><div>floatLocation :: Window -> X (ScreenId, W.RationalRect)</div><div></div><div>--.......</div><div>rr <- snd `fmap` floatLocation w</div><div> </div><div></div><div></div><div>Prelude> :i fmap</div><div>class Functor f where fmap :: (a -> b) -> f a -> f b</div><div> </div><div></div><div>It seems it is different from the definition of fmap ?</div><div></div><div>sincerely!</div></font></div></div></span></blockquote><br></div><div>As the type signature of fmap explains, it transforms a function. Specifically, it starts with a function (a -> b), and it transforms it to accept an 'a' inside a functor instead of just an a, and return a 'b' inside the same functor instead of just a b. In other words, fmap applies functions inside containers.</div><div><br></div><div>We can see from floatLocation that it returns a pair inside a container - specifically, an X container. Fmap takes snd, and transforms it to work on values inside the X.</div><div><br></div><div>So, snd has type (a,b) -> b, thus fmap snd has type f (a,b) -> f b. In this case, the type it's being applied to is X (ScreenId, W.RationalRect), so f unifies with X, a with ScreenID and b with W.RationalRect. Making snd `fmap` floatLocation w hav the type X W.RationalRect.</div><div><br></div><div>Finally, the bind into rr there takes it out of the X monad all together, getting you a W.RationalRect.</div><div><br></div><div>You may want to read this article which explains some of Haskell's abstraciton mechanisms:</div><div><br></div><div><a href="http://noordering.wordpress.com/2009/03/31/how-you-shouldnt-use-monad/">http://noordering.wordpress.com/2009/03/31/how-you-shouldnt-use-monad/</a></div><div><br></div><div>Bob</div></body></html>