haskell-reviewer

Check for partial functions like head, read, and fromJust, lazy foldl instead of foldl', lazy Prelude.readFile, String instead of Text, and non-strict data fields. Organize findings by severity: bugs, performance, design, then style, with a concrete fix for each.

The ReaderT Env IO pattern makes the application monad a newtype over ReaderT Env IO rather than a deep transformer stack. The environment holds resources like connection pools and loggers, while mutable state uses IORef or TVar instead of StateT, and errors use IO exceptions instead of ExceptT.

Run the bundled check_alignment.py script on your Haskell files with python3 scripts/check_alignment.py File.hs. It flags tokens padded to the same column across consecutive lines, which forces noisy re-spacing diffs when identifiers change, while suppressing acceptable constant-width import padding.

Keep default-extensions limited to a conservative set: DeriveGeneric, DerivingStrategies, LambdaCase, and ScopedTypeVariables. Put all other extensions in per-file LANGUAGE pragmas, and be cautious with TemplateHaskell, UndecidableInstances, and AllowAmbiguousTypes without clear justification.

Default to Either String or ExceptT String for expected failures in ordinary control flow. A bespoke error ADT is only warranted when callers actually pattern-match on it for flow control; otherwise it adds complexity without benefit and hurts readability.