This is Pie, the companion language for The Little Typer by Daniel P. Friedman and David Thrane Christiansen.
Pie is a Racket language, requiring Racket version 6.5 or newer. After installation, Racket will interpret any file beginning with #lang pie as a Pie program.
If you can't figure out what to write at some point in a Pie program, it's OK to leave behind a space to be filled out later. This corresponds to the empty boxes in The Little Typer. These TODOs are written TODO in Pie.
Pie provides additional information to DrRacket, including tooltips and other metadata. Point the mouse at a pair of parentheses, a name, or a Pie constructor or type constructor to see information about the expression.
Additionally, Pie supports the DrRacket TODO list for incomplete programs.
If you prefer an editor other than DrRacket, it may be convenient to start a Pie REPL on a command line. To do so, use the command racket -l pie -i to start Racket with the pie language in interactive mode.
Pie is available on the Racket package server. If you don't plan to make your own changes to Pie, then it is easiest to install it from there.
Click the "File" menu, and then select "Install Package...". Type pie in the box, and click the "Install" button.
Run the following command:
raco pkg install pie
Because it exists to support a book, the Pie language is finished and will not change. However, this implementation of Pie might someday acquire additional features, or it might require updates to keep up with new computers. In that case, update it as you would any Racket package.
Click the "File" menu, and then select "Install Package...". Type pie in the box, and click the "Update" button.
The command raco pkg update pie updates Pie.