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Z←(f g) R is called a Monadic HookZ ≡ R g h R
Z←L (f g) R is called a Dyadic HookZ ≡ L g h R
Z←(f g h) R is called a Monadic ForkZ ≡ (f R) g h R
Z←L (f g h) R is called a Dyadic ForkZ ≡ (L f R) g L h R
Z←(f g h ...) R
Z←L (f g h ...) R
is also defined for longer Trains
L and R are arbitrary arrays.

This clever idea from the designers of J is called Trains where a parenthesized sequence of functions (which normally would signal a SYNTAX ERROR) can be interpreted as per the above descriptions. Note that the spacing between functions is for visual purposes only — it has no effect on the interpretation.

For example,

←→    '2+3',⍎'2+3'
←→    '2+3',5
2+3 5

      avg←(+/ ÷ ⍴) defines a function that computes the average of a numeric vector.
      avg 1 2 3 4
←→    (+/ ÷ ⍴) 1 2 3 4
←→    (+/1 2 3 4) ÷ ⍴1 2 3 4
←→    10 ÷ ,4

Longer Trains are defined as follows:

(e f g h) ←→ (e (f g h))
(d e f g h) ←→ (d e (f g h))

and in general

Even length: (a b c ...) ←→ (a (b c ...))
Odd length: (a b c ...) ←→ (a b (c ...))

For more applications of this concept, see the discussion in the Learning J manual.