When comparing Growler vs Snap, the Slant community recommends Snap for most people. In the question“What are the best Haskell web frameworks for building RESTful web services?” Snap is ranked 7th while Growler is ranked 13th. The most important reason people chose Snap is:
Snap provides "snaplets". These are self-contained modules of code that provide additional functionality without muddling the core library. Having these resources saves time when developing code.
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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 Provides modules for changeable functionality
Snap provides "snaplets". These are self-contained modules of code that provide additional functionality without muddling the core library. Having these resources saves time when developing code.
Cons
Con Potentially unstable API
Growler is still at an experimental stage and users are warned that breaking changes may happen between versions.
Con Outdated documentation
The comprehensive Snap documentation is currently outdated (last activity appears to have been 2013), though Snap itself is under active development. Note that this primarily affects tutorials and guides; the API documentation is generated automatically and remains current.