Write You a Haskell Stephen Diehl

data PlatonicSolid
= Tetrahedron
| Cube
| Octahedron
| Dodecahedron
| Icosahedron
deriving (Show, Eq, Ord, Read)
example = show Icosahedron
example = read ”Tetrahedron”
example = Cube == Octahedron
example = sort [Cube, Dodecahedron]
IO
A value of type IO a is a computation which, when performed, does some I/O before returning a value
of type a. e notable feature of Haskell is that IO is still functionally pure; a value of type IO a is
simply a value which stands for a computation which, when invoked, will perform IO. ere is no way
to peek into its contents without running it.
For instance, the following function does not print the numbers 1 to 5 to the screen. Instead, it builds
a list of IO computations:
fmap print [1..5] :: [IO ()]
We can then manipulate them as an ordinary list of values:
reverse (fmap print [1..5]) :: [IO ()]
We can then build a composite computation of each of the IO actions in the list using sequence_,
which will evaluate the actions from left to right. e resulting IO computation can be evaluated in
main (or the GHCi repl, which effectively is embedded inside of IO).
>> sequence_ (fmap print [1..5]) :: IO ()
1
2
3
4
5
>> sequence_ (reverse (fmap print [1..5])) :: IO ()
5
4
3
2
1
23