Mapping over the range and pattern matching operators

30 Jan 2021 ⇐ Notes archive

(This is an entry in my technical notebook. There will likely be typos, mistakes, or wider logical leaps — the intent here is to “let others look over my shoulder while I figure things out.”)

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Two tricks for the operator and point-free-leaning crowd:

1. To quickly form ranges from a sequence of indices, you can use zip(indices, indices.dropFirst()).map(..<) (or subbing in the closed range operator — the one-sided variants are ambiguous in the point-free style)¹.

    let odds = [1, 3, 5, 7]
   
    zip(odds, odds.dropFirst()).map(..<) // ⇒ [(1..<3), (3..<5), (5..<7)]
   

2. And tangentially, to test if parallel sequences of ranges and values match against one another.

    // (…continuing from above.)
   	
    let odds = [1, 3, 5, 7]
    let oddRanges = zip(odds, odds.dropFirst()).map(..<)
   	
    let evens = [2, 4, 6]
    zip(oddRanges, evens).map(~=) // ⇒ [true, true, true]
   

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