On Fri, 2015-03-06 at 11:18 +0100, Urs Liska wrote: > Am 06.03.2015 10:57, schrieb Richard Shann: > > On Fri, 2015-03-06 at 08:25 +0100, Urs Liska wrote: > >> I find > >> > >> %% switch on debugging. > >> #(if (and #t (defined? 'set-debug-cell-accesses!)) > >> (set-debug-cell-accesses! 5000)) > >> > >> (found in init.ly) > >> > >> strange. > >> Wouldn't it be better ot simply remove the 'and' construct? > >> #t evaluates to #t always, isn't it? > > The terminology used in the manual is "#t evaluates to true". (And, > > indeed #f is the only thing that evaluates to false). I think I then > > found it tricky to find where in the manual this concept (true/false) > > was explained, but came to understand it applies to the semantics of > > "if" and "while". > > The point in this is that if an expression evaluates to "something" this > "something" is considered #t
no, it isn't "considered to be #t". It is considered to be true (that is, it is considered not to be #f). You can tell this by testing (eq? expr #t) will be false if expr is anything true except for #t which is one of the (many) true things in Scheme. > with regard to conditionals but the return > value is "something". > > So > > #(if (get-something-that-returns-an-empty-or-non-empty-list) > (do-something) > (do-nothing)) > #(let > ((result (get-something-that-returns-an-empty-or-non-empty-list))) > (if result > (display result))) > > In the second case result can be passed to "if" but holds the result of > the procedure that can then be used. > This can also be made fruitful in 'and' or 'or' constructs that will > hold a result in the end - instead of only the boolean result of the > condition. > > > > >> Or is this one more Scheme subtlety I don't get? > > so, I don't think so - the commonest cause is where someone starts with > > (if (and Debug (defined? 'xxx)) > > > > and then decides not to have a separate Debug flag and replaces it in > > bulk with #t > > > > Richard > > > > OK, I think this means that could be removed. certainly! Richard _______________________________________________ lilypond-user mailing list lilypond-user@gnu.org https://lists.gnu.org/mailman/listinfo/lilypond-user
