Readability

Maintaining readability of APL can take a special effort. It is easy to write very dense code, and the mathematical look of APL can encourage usage of single-letter names. Traditionally, APLers use the term pornography to describe code that is hard to read, or uses unusual constructs. Code golf often results in pornographic code.

Examples

Gilman & Rose

In APL – an Interactive Approach, the authors the authors describe the following code, which computes the correlation coefficient, as “almost pornographic”:

r←(+/x×y)÷((+/(x←x-(+/x)÷⍴x)*2)×+/(y←y-(+/y)÷⍴y)*2)×.5

IBM

The APL2 Idiom list includes the following entry:

X←'line1',0⍴Y←'line2'

⍝ Pornography. Combining two lines into one.

This was a common technique before Left was added to the language:

X←'line1' ⊣ Y←'line2'

The Diamond statement separator (⋄) provides an alternative means of inlining multiple statements:

Y←'line2' ⋄ X←'line1'

Note that in all these cases, Y is assigned first.

Morten Kromberg

Morten Kromberg asked one of his colleagues to “Please avoid this kind of pornography:”

ns(⍎container.⎕NS)←⍬

Avoiding the unusual modified assignment (using the 2-train ⍎⎕NS as modifying function) helps:

ns←ns container.(⍎⎕NS) ⍬

Finally, splitting the 2-train apart makes it even clearer:

ns←⍎ns container.⎕NS ⍬

A new namespace, with the original value of ns as name, is created inside container and the character representation '#.container.ns' is returned from ⎕NS to ⍎ which evaluates the name to a reference, that in turn replaces the previous value of ns.