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lastLE cleanup

Remember lastLE? Back when we were trying to determine how many digits/bits it takes to represent a non-negative Integer, we used it to avoid copying a prefix of a list just to grab the value at its end.

lastLE :: Integer -> [Integer] -> Maybe (Integer, Int)

lastLE n xs =
    let lastLE' xs prevVal prevIndex
           | head xs <= n = lastLE' (tail xs) (head xs) (prevIndex + 1)
           | otherwise    = if prevIndex < 0 then Nothing
                                             else Just (prevVal, prevIndex)
    in lastLE' xs (-1) (-1)

It still bugs me. First, the mixed guards and if/else are less than idiomatic Haskell:

lastLE n xs =
    let lastLE' xs prevVal prevIndex
           | head xs <= n  = lastLE' (tail xs) (head xs) (prevIndex + 1)
           | prevIndex < 0 = Nothing
           | otherwise     = Just (prevVal, prevIndex)
    in lastLE' xs (-1) (-1)

Second, the head and tail are clumsy:

lastLE n xs =
    let lastLE' (x:xs) prevVal prevIndex
 …

Small things...

Good grief... I've let this go for a month. (UPDATE: No, I haven't. It helps to read the dates carefully.)

An option in pattern matching that I wasn't fully aware of: you can use _ when you don't care about what is there, and have no need to refer to it. I guess I did use it on the ByteString readInteger that returns Maybe(Integer, ByteString):

process line = case map B.readInteger (B.words line) of
               [Just (m, _), Just (n, _)] -> numXs m n
               _                          -> -1


But I didn't realize that I could use it in

ysInRange m n d
    | nVal == n = nPos - mPos + 1
    | otherwise = nPos - mPos
    where (_,    mPos) = findFirst' m
          (nVal, nPos) = findFirst' n
          findFirst' x = case findFirst (Ge x, Any) (dDigitYTree d) of
                             Just (Max i, Max j) -> (i, j)
                             Nothing             -> (tenToThe d, numNDigitYs d)


because we need only check for an exact match at t…