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module Y2017.M01.D11.Solution where
Continuing along with Haskell Pitfalls from @bitemyapp, let's look at partial
functions in the Prelude, particularly for the hated, evil LIST.
Actually I'm a LISP-guy, so, by definition, I'm a List-guy. SOMEtimes I use
list when PERHAPS I could use something else ...? Perhaps because it works
so well and so easily for so many problems? Perhaps?
But I digress.
So, let's look at head, tail, init and last.
These are partial functions because they fail for []:
*hask> tail []
*** Exception: Prelude.tail: empty list
So, that's the problem. Another problem is with the Maybe-type when you call
fromJust when there's no JUSTice:
*hask> fromJust Nothing
*** Exception: Maybe.fromJust: Nothing
But the solution for that is not to use fromJust, but use other maybe-functions
(e.g.: fromMaybe or maybe)
So, I'll just skip the fromJust-issue, and focus today only on List-functions.
So, head, tail, init, and last are partial functions on List (that can be []).
How do we solve this partialiality issue for these List functions?
Well, have these functions work only an a new type: NonEmptyList
OR have these functions unpartialized, or totalled, by returning Maybe-values?
What to do?
Well, introducting a new type NonEmptyList is going to have a LOT of buy-in,
isn't it, like the wholesale adoption of Text and ByteString (ByteArray, more
like) over the 'inefficient' String, right?
Well, it is one way to go.
The other way is to en-maybe-ify these functions. Let's do that:
mbHead, mbLast :: [a] -> Maybe a
mbHead [] = Nothing
mbHead (h:t) = Just h
mbLast [] = Nothing
mbLast list@(_:_) = Just (last list) -- a total fn that uses a partial fn
mbTail, mbInit :: [a] -> Maybe [a]
mbTail [] = Nothing
mbTail (h:t) = Just t
mbInit [] = Nothing
mbInit list@(_:_) = Just (init list) -- sounds legit
Okay, with your definitions of the above TOTAL functions, what are the values
mbHead, mbLast, mbTail, mbInit
for the following values:
nada, uno, buncha :: [Int]
nada = []
uno = [1]
buncha = [1..10]
*Y2017.M01.D11.Solution> [mbHead, mbLast] <*> [nada, uno, buncha] ~>
[Nothing,Just 1,Just 1,
Nothing,Just 1,Just 10]
*Y2017.M01.D11.Solution> [mbTail, mbInit] <*> [nada, uno, buncha] ~>
[Nothing,Just [],Just [2,3,4,5,6,7,8,9,10],
Nothing,Just [],Just [1,2,3,4,5,6,7,8,9]]
-- There! Total functions on List! Feel better now?