When comparing Growler vs Spock, the Slant community recommends Spock for most people. In the question“What are the best Haskell web frameworks for building RESTful web services?” Spock is ranked 2nd while Growler is ranked 13th. The most important reason people chose Spock is:
Spock parses request URLs into well-typed data without requiring your routes to be declared at the type level.
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Pros
Pro Clean, simple route building
Growler improves on Scotty's route building functions, allowing hierarchical declarations rather than requiring each endpoint to be fully specified.
Pro Type-safe, function-level routes
Spock parses request URLs into well-typed data without requiring your routes to be declared at the type level.
Pro Small but complete
Unlike scotty it manages to be small and, at the same time, includes all the necessities like sessions and db pool.
Cons
Con Potentially unstable API
Growler is still at an experimental stage and users are warned that breaking changes may happen between versions.