It takes a string representing train as right argument. all lowercase letters are functions, all uppercase letters are constants. Left arg is valence when applied.
@LeakyNun On the other hand, it is often easier to express things explicitly (especially for newcomers), and explicit programming is basic to APL, while awkward in J.
Some of them prohibit us from telling who they are so their competition won't find out what gives them their advantage, but I can tell you that one of the biggest oil companies uses APL to manage their "chemicals' recipes".
Also one of the biggest financial services technology providers run everything in APL.
One I am allowed to mention: Volvo's heavy-engine division (trucks, ships, etc.) Their entire production is managed by one large APL program.
It goes pair in pair with ¨. For instance, I had an array of arrays containing indices and had to calculate which elements the indices mapped to. I needed to use ⌷, but it requires the indices to be the left argument, so the only tool that I could use to swap the arguments around was the tildiaresis
@KritixiLithos Practical, yes, but what it does is really simple, and it can always be replaced with a dfn: A f⍨ B ←→ A{⍵f⍺}B. Only for trains, is it essential, as it allows constants as right tine: Square←{⍵*2} ←→ Square←2*⍨⊢, although it can often be substituted with composition: Square←*∘2.
