Dyalog '13: Train Spotting in Version 14.0

John Scholes

The Function Train is a new syntax construct, supported in Version 14.0, which assigns meanings to expressions of juxtaposed functions in isolation. For example, the expression x(f+g)y, where the parentheses surround a fork, is equivalent to (x f y)+(x g y); and x(f g)y, where the inner expression is pronounced f atop g, is equivalent to f(x g y). The J language has supported these constructs for many years and they have been shown to add significant expressive power to the language, simplifying the translation of mathematical expressions into executable code through tacit programming. This presentation demonstrates the benefit of trains and gives examples of how they occur naturally when coding APL.