repl is a simple command line interface. It uses haskeline for input editing, which is similar to readline, only with different bugs. Haskeline can be configured in ~/.haskeline, but basically what you want in there is editMode: Vi and historyDuplicates: IgnoreConsecutive. Full documentation is at http://trac.haskell.org/haskeline.

The repl accepts haskell which is evaluated in a Cmd.Cmd.CmdL monad, which is just a synonym for CmdT IO a. The result is displayed directly if it's a String, or otherwise given to show and pretty printed. There's also a pp function which will format the result with the Util.Pretty.Pretty typeclass, which is easier to read but may omit data.

The Cmd.Repl.Environ module is in scope, which in turn imports just about everything interesting. Notably, it imports Cmd.Repl.Global unqualified, which provides a basic vocabulary of functions, and all the Cmd.Repl.L* modules, which provide functions intended to be used from the REPL. In case you're wondering about the L prefix, it's because the directory used to be called Lang, and I got used to the L. And there has to be some prefix, otherwise all those module names clash with other module names.

The REPL will load all the modules when the app starts. Like ghci, if the .o files are missing or out of date, it will load modules as bytecode. This is fine if it's a few modules in Cmd/Repl, but if some low level module has been touched it will want to byte-compile everything, and usually crash on the spot. Not sure why. Anyway, make sure you did a fresh build. If you just modified high-level modules like the ones in Cmd/Repl, you can type :r in the REPL and it will reload them.

The REPL also has a simple macro feature to make it easier to write IDs. @name will become (*Id "ns/name") where * is BlockId, RulerId, TrackId, or ViewId, whatever is appropriate, and ns is the Ui.UiConfig.config_namespace. @"a b c" will let you put spaces or symbols or other non-letter non-dash letters in.

You can do tons of stuff from the REPL, and in fact most operations can only be done from the REPL. Read through the Cmd/Repl directory to get some ideas. If an expression is too large to type directly at the REPL, you can edit a Cmd/Repl module, or open a new module in Local/Repl, and use :r to reload.