[Haskell-cafe] Lazy Lists and IO

[Andrew Coppin]

Chaddaï Fouché wrote:
> 2007/7/11, Andrew Coppin <andrewcoppin at btinternet.com>:
>>
>> Ouch! That's gotta sting...
>>
>> I wasn't aware that this function was so leathal. I use it constantly
>> all the time...
>>
>
> It isn't that "lethal" usually. It's only because he was using it on
> an infinite stream that it hurt so much... If you use it on a normal
> stdin or a hGetContents on a file it will be fine (though you will
> lose the advantage of its laziness, for example constant memory
> treatment).
>

Loose lazyness?

Oh wait - you mean the inadvertent length thing?



We already have null, but how about a standard function in Data.List for 
testing whether the length is longer than some upper limit?

  lengthUpTo :: Int -> [x] -> Int
  lengthUpTo n = length . take n

  shorterThan :: Int -> Bool
  shorterThan n = (n >) . lengthUpTo (n+1)

  longerThan :: Int -> Bool
  longerThan n = (n <) . lengthUpTo n

  lengthEquals :: Int -> Bool
  lengthEquals n = (n ==) . lengthUpTo (n+1)

(Perhaps need better names...)