So... First, '' is an empty list of characters.
Type: literal Rank: 1 Shape: ,0 Data: empty In other words an empty string. And, f is a verb. f'' gives us the result of its monadic definition applied to the empty string argument. And, since f will be a constant function it's always going to give us the same result. Here's the definition of Y: Y=: 3 :0 (Y bind y`'')&(y`:6)`'' ) So, in almost_fibonacci (when it's being handled through Y), f is going to be the constant function Y bind (almost_fibonacci`'') and f'' is going to be the gerund resulting from Y almost_fibonacci`'' and (f'')`:6 is going to be the same as (Y almost_fibonacci`'')`:6 -- it's a function to compute a fibonacci number. Also note that `'' is something very different -- it's a suffix which converts a verb into a gerund (a noun which represents that verb). In other words, f is a verb which when run gives us the result of Y applied to the gerundial form of almost_fibonacci. We could not compute this result directly in Y (because it's a part of the result of Y), so instead of providing the result directly we provided a gerund that would let us calculate this result. (And a gerund is the most direct approximation of a lambda that I have found in J -- and a trailing `:6 is the syntactic mechanism in J which gives an effect like a lambda being at the head of a list, in a lisp dialect). So... the result of f'' is a gerund which can be used to find a fibonacci number. Does this make sense? Thanks, -- Raul On Wed, Jun 13, 2012 at 2:26 AM, Steven Taylor <tayl...@gmail.com> wrote: > thanks for the post. I have wondered about recursion in J before. I'd > like to dig into your example a little more. > > Could you elaborate on *f'' gives us a gerund that can handle one more > level of recursion > * > Why the double single quote? > > thanks, > -Steven > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
