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什么是skolems？
eek！GHCi在我的代码中找到了Skolems！ ... Couldn't match type `k0' with `b' because type variable `b' would escape its scope This (rigid, skolem) type variable is bound by the type signature for groupBy :: Ord b => (a -> b) -> Set a -> Set (b, [a]) The following variables have types that mention …