The next generation of ghcid, a ghci-based file watcher
and recompiler. ghciwatch watches your modules for changes and reloads them in
a ghci session, displaying any errors.
When we started working on ghciwatch, ghcid suffered from some significant
limitations. In particular, ghcid couldn't deal with moved or deleted
modules, and wouldn't detect new directories because it can't easily update
the set of files being watched at runtime. We've also seen memory
leaks requiring multiple restarts per day. Due to the ghcid codebase's
relatively small size, a reimplementation seemed like a more efficient path
forward than making wide-spanning changes to an unfamiliar codebase.
Rust makes it easy to ship static binaries. Rust also shares many features with
Haskell: a Hindley-Milner type system with inference, pattern matching,
and immutability by default. Rust can also interoperate with
Haskell, so in the future we'll be able to ship ghciwatch as a
Hackage package natively. Also, Rust's commitment to stability makes coping
with multiple GHC versions and GHC upgrades easy. Finally, Rust is home to the
excellent cross-platform and battle-tested notify library, used to
implement the watchexec binary and cargo-watch, which solves a
lot of the thorny problems of watching files for us.
Recompiling a project when files change is a fairly common development task, so
there's a bunch of tools with this same rough goal. In particular,
watchexec is a nice off-the-shelf solution. Why not just run
watchexec -e hs cabal build? In truth, ghciwatch doesn't just recompile the
project when it detects changes. It instead manages an interactive ghci
session, instructing it to reload modules when relevant. This involves a fairly
complex dance of communicating to ghci over stdin and parsing its stdout, so
a bespoke tool is useful here.