Fork

<=   =>

A train of three verbs is a fork and is evaluated as follows:

  (f g h) y	evaluates as	(f y) g (h y)
x (f g h) y	evaluates as	(x f y) g (x h y)

A simple example of a fork is the sequence of three verbs +/ % # . The / adverb takes its left argument + and returns a verb, so there is a sequence of three verbs. Let's examine the use of this fork monadically.

   (f  g h) y	evaluates as	(f  y) g (h y)
   (+/ % #) y	evaluates as	(+/ y) % (# y)

This can be read as: sum over the argument divided by the count of the argument. This is the definition of the mean or average.

   (+/ % #) 5 9 12
8.66667
   (+/ % #) i.9
4

You can now define mean as a fork with +/ % # .

   mean =. +/ % #
   mean i.9
4

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