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This merge request adds support for higher-rank polymorphism to the frontend.

The extension of the existing type system is based on the paper Practical type inference for arbitrary-rank types also implemented by Haskell. The new type system is still predicative. An instantiation of type variables with higher-rank types is not allowed. But Curry programmers use the ($)-operator so much, to avoid writing parentheses, that the extended type inference has an ad-hoc special case for x $ y (just like Haskell) that allows it to do type inference for x $ y, even when impredicative polymorphism is needed.

Depends on curry-base!9 (merged)

Work that needs to be done:

• Convert forall types to AbstractCurry
• Add RankNTypes to the known language extensions of the frontend
• Convert the TypeScheme data type into a forall type
• Allow type constraints nested within type expressions
• Implement the new typing rules
• Adapt the checks and transformations
• Detect impredicative polymorphism
• Correct typing of higher-rank functions with predicates
• Add tests for RankNTypes to the test suite

Bugs that need to be fixed:

• Prelude.!!: index too large (#40 (closed))
• Compilation error (#42 (closed))
• Problems with additional constraints for functions inside InstanceDecls (#46 (closed))
• Subsumption is not working correctly (#48 (closed))