Justin Bailey wrote:
-- Determines if the length of the strings in the list is longer than the given
-- count. If not, amount the list falls short is returned. Otherwise,
-- -1 indicates the prefix list is at least that long. If the count is zero and
-- the list is empty or just null strings, -1 is also returned.
>
prefixesAtLeast :: Int -> [S.ByteString] -> Int
While that doesn't help your stack overflow problem, it's not very
haskellish to return magic numbers. A Maybe type is more appropriate here.
prefixesAtLeast !0 !ss
| null ss = 0
| all S.null ss = 0
| otherwise = -1
prefixesAtLeast !n !ss = prefixesAtLeast' n ss
where
prefixesAtLeast' !n ss
| n < 0 = -1
| null ss = n
| otherwise =
let (!s : (!rest)) = ss
in
prefixesAtLeast' (n - (S.length s)) rest
Extracting the head and tail of ss with a let statement could lead to
a huge unevaluated expression like
rest = tail (tail (tail (...)))
but the null test are likely to force it. Also note that the case n = 0
is quite rare. In any case, I'd write the function as
lengthExcess :: Int -> [S.ByteString] -> Maybe Int
lengthExcess n ss
| n <= 0 = Nothing
| otherwise = case ss of
[] -> Just n
(s:ss) -> lengthExcess (n - S.length s) ss
Note the that the function name is chosen to mnemonically match the
result type Maybe Int, i.e. "the excess is Just 5 characters" or "the
excess is Nothing".
Regards,
apfelmus
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