39 lines
933 B
Text
39 lines
933 B
Text
|
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
|
||
|
|
---
|
||
|
|
|
||
|
|
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
|