Ian wrote:
> Hands up who understands ~help/dictionary/d212.htm
$: is used for (anonymous) recursion. For example, where you might've
written:
fact =: * fact@:<:^:(1&<)
fact 5
120
using $:, you could've omitted the verb-naming step (which is incidental to
its function, or at least you'd like that to be true):
( * $:@:<:^:(1&<) ) 5
120
but note: $: refers to the _longest_ verb that contains it. So whereas:
factMas2 =: 2 + fact
factMas2 5
122
vs:
(2 + ( * $:@:<:^:(1&<) )) 5
532
All of a sudden, $:'s scope has broadened to include the 2&+ (on _every_
invocation, including the recursive ones). Of course, you could fix this by
manually limiting the scope of $:
fact1 =: * $:@:<:^:(1&<)
fact1Mas2 =: 2 + fact1
fact1Mas2 5
122
... but that kind of defeats the purpose a bit. Also, if anyone ever
decides to apply f. and it hits your $:-verb, it'll fix you good:
fact1Mas2 f. 5 NB. Theoretically identical to line above
3
There are ways around this (e.g. 2 + 3 : '( * $:@:<:^:(1&<) ) y' ) but
none is entirely satisfying.
-Dan
PS: f. should wrap all verbs in parens before quoting them in an explicit
context.
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