Programming Praxis

Clustering is the process of collecting in groups all of the items from an input collection that share some common feature; for instance, the GROUP BY operator of SQL performs clustering. We will define cluster(proc, lt?, lst) as a function that takes an input list and returns a list of lists; proc computes a signature of each item in the input list, and each sub-list in the output list contains all those elements of the input list with identical signatures, with sub-lists in increasing order of signature according to lt?. The type of cluster is (α → β) × (β × β → boolean) × (list α) → (list (list α)).

Your task is to write the function cluster. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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