Chapter 1: Functional Programming
===

Exercise 1.2
---

Trawling through Data.List we discovered the function

> uncons :: [a] -> Maybe (a, [a])

of whose existence we were quite unconscious. Guess the definition of uncons.

> uncons [] = Nothing
> uncons (hd : tl) = Just (hd, tl)

Exercise 1.3
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The library Data.List does not provide functions

> wrap :: a -> [a]
> unwrap :: [a] -> a
> single :: [a] -> Bool

for wrapping a value into a singleton list, unwrapping a singleton list into
its sole occupant, and testing a list for being a singleton. This is a pity,
for the three functions can be very useful on occasions and will appear a
number of times in the rest of this book. Give appropriate definitions.

> -- return should work here too
> wrap x = [x]
>
> -- TBH unwrap should be returning Maybe since otherwise this throws an
> -- exception on unmatched cases
> unwrap [x] = x
>
> single [x] = True
> single _ = False