PiLisp: pl> Macro

PiLisp's REPL wraps everything you submit to it with an invocation of its pl> macro.

The pl> macro expects expressions to be separated by the pipe | symbol. The forms between pipe separators are wrapped in an implicit pair of parentheses, meaning that they are invoked, and the return value of each pipe-delimited expression is threaded as the last argument to the next expression:

pl> bindings | keys | sort | take 10
(
  *
  *1
  *2
  *3
  *e
  *pilisp-version*
  +
  -
  ->
  ->>
)

The initial form is invoked only if it is a function or a symbol that resolves to one, so that other types of values can be entered directly:

pl> [0 1 2 3 4 5 6 7 8 9] | map (partial * 3) | filter even?
(
  0
  6
  12
  18
  24
)

Finally, if you want to have the previous expression's value placed in a non-final position, you can use the $ symbol:

pl> "hello" | dart/String.toUpperCase | str $ " world!"
"HELLO world!"

A desire—at the REPL—to write programs in this style by default was part of my initial inspiration for building PiLisp's predecessor sc which had this syntax hard-coded into the language implementation. One of the most satisfying moments of PiLisp's development was implementing macro support to the point where I could write pl> as a macro in PiLisp itself.

This syntactic approach of pl> presents problems when a program spans multiple lines, but at an interactive REPL, the ability to append forms and further refine a program by pressing the up key followed by | leverages years of CLI/shell muscle memory and results in a low-friction experience for small experiments. It also naturally leads to thoughts about mirroring the system shell in other ways, e.g., by supporting a notion of "current directory" as Ruby's pry does. I did this for sc, allowing one to "step into" a Shortcut story, epic, iteration, milestone, etc, and PiLisp currently supports a rudimentary facility for using cd to "step into" data at the REPL.

In my own Clojure development, I have moved away from using interactive REPLs directly when writing larger programs, preferring to interact with the REPL via code in files. For code bases with multiple files, dependencies, and a lot of different classpath and conceptual contexts to navigate, I find this workflow superior to typing at the Clojure REPL directly.¹

But PiLisp is designed for smaller programs. A fast-to-start Lisp environment that provides a CLI REPL by default, with access to the Dart ecosystem when your experiments need to expand, and that can be compiled to a standalone executable by the built-in toolchain² has filled several gaps for me that Clojure on the JVM alone has not been able to fill.

Tags: clojure programming-language pilisp cli dart macro

Functions, Messages, and Words

PiLisp