When comparing Spock vs Yesod, the Slant community recommends Yesod for most people. In the question“What are the best Haskell web frameworks for building RESTful web services?” Yesod is ranked 1st while Spock is ranked 2nd. The most important reason people chose Yesod is:
While not required, Yesod offers templating through a Shakespearean family of languages to produce page code.
Specs
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Pros
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.
Pro Offers templating for type-safe, well-formed content
While not required, Yesod offers templating through a Shakespearean family of languages to produce page code.
Pro Uses type-safe URLs
Ensures that data provided by the URL is type-safe. This means that data in the URL has a definitive type.
Pro Can be used without knowing much Haskell
While Yesod is written in Haskell, developers can achieve basic functionality without much investment in the language.
Cons
Con Is hard to customize
Learning how Yesod works internally is hard. It is a large framework with complicated components. Finding the appropriate code and understanding how it contributes to the framework is difficult, meaning developers will struggle.
Con Too much template haskell
You actually want to code in haskell and not some DSL with "magic" hidden under the bonnet.