8TYPE CLASSES
We have seen functions that have a concrete type, such as not, which takes a Bool as input and returns a Bool as output. We have also seen functions with types that use a type variable to express that they work with all types, such as head, which takes a list of any type and returns the first element. Use of a type variable expresses commonality over all types.
Between the extremes of functions that work with a single type and functions that work with all types is a need to express a more limited commonality among types that does not extend to all types. For example, we would like addition to be available for numeric types like ...
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