---
title: Higher-Order Programming II
---

Spotting Patterns In The "Real" World
=====================================

It was all well and good to see the patterns in tiny "toy" functions.
Needless to say, these patterns appear regularly in "real" code, if only
you know to look for them. Next, we will develop a small library that
"swizzles" text files.

1. We will start with a beginner's version that is 
   riddled with [explicit recursion](swizzle-v0.html).

2. Next, we will try to spot the patterns and eliminate 
   recursion using [higher-order functions](swizzle-v1.html).

3. Finally, we will parameterize the code so that we can
   both "swizzle" and "unswizzle" without having to duplicate code.

Exercise
--------

Needless to say, the code can be cleaned up even more, 
and I encourage you to do so. For example, rewrite the 
code that swizzles `Char` so that instead of using
association lists, it uses the more efficient `Map k v` 
(maps from keys `k` to values `v`) type in the standard 
library module `Data.Map`.

Recursive Types
===============

Recall that Haskell allows you to create brand new data types like this one
from [lecture 1](lec1.html).

> data Shape  = Rectangle Double Double 
>             | Polygon [(Double, Double)]

Values of this type are either two doubles *tagged* with `Rectangle` 

~~~~~{.haskell}
ghci> :type (Rectangle 4.5 1.2)
(Rectangle 4.5 1.2) :: Shape
~~~~~

or a list of pairs of `Double` values *tagged* with `Polygon`

~~~~~{.haskell}
ghci> :type (Polygon [(1, 1), (2, 2), (3, 3)])
(Polygon [(1, 1), (2, 2), (3, 3)]) :: Shape
~~~~~

One can think of these values as being inside special "boxes"
with the appropriate tags.

![Datatypes are Boxed-and-Tagged Values](/static/lec4_boxed.png)

However, Haskell (and other functional languages), allow you to define
datatypes *recursively* much like functions can be defined recursively.
For example, consider the type

> data IntList = IEmpty 
>              | IOneAndMore Int IntList
>              deriving (Show)

(Ignore the bit about `deriving` for now.) What does a value of type `IntList`
look like? As before, you can *only* obtain them through the constructors.
Here, we have the constructor `IEmpty` takes no arguments and returns an
`IntList`, that is

~~~~~{.haskell}
ghci> :type IEmpty 
IEmpty :: IntList 
~~~~~

Now, that we have at least one value of type `IntList` we can make more by
using the other constructor

> is1 = IOneAndMore 1 IEmpty
> is2 = IOneAndMore 2 is1
> is3 = IOneAndMore 3 is2

and so on. Suppose we ask Haskell to *show* us `is3` we get

~~~~~{.haskell}
ghci> is3
IOneAndMore 3 (IOneAndMore 2 (IOneAndMore 1 IEmpty))
ghci> :type is3
is3 :: IntList
~~~~~

However, the presence of recursion does not change what the values really
are, now you simply have boxes within boxes.

![Recursively Nested Boxes](/static/lec4_nested.png)

Of course, there is no reason why the type definition must have only one
*recursive* occurrence of the type. You can encode general *trees* like 

> data IntTree = ILeaf Int
>              | INode IntTree IntTree
>              deriving (Show)

Here, each value is either a simple *leaf* which is a box containing an
`Int` labeled `ILeaf`, such as each of

> it1  = ILeaf 1 
> it2  = ILeaf 2

or an *internal node* which is a box containing two trees, a left and right
tree, and marked with a tag `INode`, such as each of

> itt   = INode (ILeaf 1) (ILeaf 2)
> itt'  = INode itt itt
> itt'' = INode itt' itt'

Now, if we ask Haskell

~~~~~{.haskell}
ghci> itt'
INode (INode (ILeaf 1) (ILeaf 2)) (INode (ILeaf 1) (ILeaf 2))

ghci> :type itt''
itt' :: IntTree
~~~~~

Needless to say, you can have multiple branching factors, for example
you can define [2-3 trees](http://en.wikipedia.org/wiki/2-3_tree) over
integer values as 

> data Int23T = ILeaf0 
>             | INode2 Int Int23T Int23T
>             | INode3 Int Int23T Int23T Int23T
>             deriving (Show)

An example value of type Int23T would be

> i23t = INode3 0 t t t
>   where t = INode2 1 ILeaf0 ILeaf0

which looks like 

![Integer 2-3 Tree](/static/lec4_int23t.png)

and has the type

~~~~~~{.haskell}
ghci> :type i23t 
i23t :: Int23T
~~~~~~{.haskell}


Parameterized Types
===================

We could go on and define versions of `IntList` that stored, say, `Char`
and `Double` values instead.

> data CharList   = CEmpty 
>                 | COneAndMore Char CharList
>                 deriving (Show)
>
> data DoubleList = DEmpty 
>                 | DOneAndMore Char CharList
>                 deriving (Show)

and we could go on and on and do the same for trees and 2-3 trees and 
so on. But that would be truly lame, because we would (mostly) repeating 
ourselves. This looks like a job for abstraction! 

The song remains the same: as when finding abstract *computation* 
patterns, we can find abstract *data* patterns by identifying the 
bits that are different and turning them into parameters. Here, the
bit that is different is the underlying *base* data stored in each
box of the structure. So, we will turn that base datatype into a
*type parameter* that is passed as input to the type constructor.

> data List a = Empty 
>             | OneAndMore a (List a) 
>             deriving (Show)

Now, as before, we may define each of the types as simply *instances* 
of the above parameterized type

~~~~~{.haskell}
type IntList    = List Int
type CharList   = List Char
type DoubleList = List Double
~~~~~

Similarly, it is instructive to look at the types of the constructors 

~~~~~{.haskell}
ghci> :type Empty 
List a
~~~~~

That is, the `Empty` tag is a value of *any* kind of list, and

~~~~~{.haskell}
ghci> :type OneAndMore 
a -> List a -> List a
~~~~~

That is, the constructor two arguments: of type `a` and `List a` and
returns a value of type `List a`. Here's how you might construct values
of type `List Int` (note that you can use the binary constructor function
in infix form)

> l1 = OneAndMore 'a' `OneAndMore` Empty
> l2 = OneAndMore 'b' `OneAndMore` l1 
> l3 = OneAndMore 'c' `OneAndMore` l2 

Of course, this is pretty much how the "built-in" lists are defined
in Haskell, except that `Empty` is called `[]` and `OneAndMore` is 
called `:`.

One can use parameterized types to generalize the definition of the 
other data structures that we saw. For example, trees

> data Tree a   = Leaf a 
>               | Node (Tree a) (Tree a) 
>               deriving (Show)

and *2-3* trees

> data Tree23 a = Leaf0  
>               | Node2 (Tree23 a) (Tree23 a)
>               | Node3 (Tree23 a) (Tree23 a) (Tree23 a)
>               deriving (Show)

Of course, there is no reason to limit ourselves to a single type
parameter! We might define a type 

~~~~~{.haskell}
data Map k v = Emp 
	     | Bnd k v (Map k v) (Map k v)
             deriving (Show)
~~~~~

What do you think such a type would be useful for?

Kinds
-----

The `Tree a` corresponds to trees of values of type `a`. If `a` is the
*type parameter*, then what is `Tree` ? A *function* that takes a type `a`
as input and returns a type `Tree a` as output! But wait, if `List` is a
"function" then what is its type? A *kind* is the "type" of a type.

~~~~~{.haskell}
ghci> :kind Int
Int :: *
ghci> :kind Char
Char :: *
ghci> :kind Bool
Bool :: *
~~~~~

Thus, `List` is a function from any "type" to any other "type, and so 

~~~~~{.haskell}
ghci> :kind List
List :: * -> *
~~~~~

We will not dwell too much on this now. As you might imagine, they allow 
for all sorts of abstractions over how to construct data.
See this for more information about [kinds](http://en.wikipedia.org/wiki/Kind_(type_theory)). 


Computing Over Trees
====================

We have (and will continue) to see that trees are crucial pillar upon which
many data structures are built in Haskell. The workhorse, lists, are infact
a kind of tree with exactly one child. Lets write some functions over
trees.

First, here is a function that computes the *height* of a tree.

~~~~~{.haskell}
height :: Tree a -> Int
height (Leaf _)   = 0
height (Node l r) = 1 + max (height l) (height l)
~~~~~


The readability of the above, makes an English description redundant!
Good old recursion. You have a base case for the `Leaf` pattern (namely
`0`) and a recursive or inductive case for the `Node` pattern (namely the 
larger of the recursively computed heights of the left and right subtrees.)
Lets give it a whirl

> st1 = Node (Leaf "cat")    (Leaf "doggerel")  
> st2 = Node (Leaf "piglet") (Leaf "hippopotamus") 
> st3 = Node st1 st2

~~~~~{.haskell}
ghci> height st1
1

ghci> height st3
2
~~~~~

How do we compute the *number* of leaf elements in the tree?

~~~~~{.haskell}
size :: Tree a -> Int
size (Leaf _)   = 1
size (Node l r) = (size l) + (size l)
~~~~~


How about a function that gathers all the elements that occur 
as leaves of the tree:

~~~~~{.haskell}
toList :: Tree a -> [a] 
toList (Leaf x)   = [x]
toList (Node l r) = (toList l) ++ (toList r)
~~~~~


Computation Pattern: Tree Fold 
------------------------------

Did you spot the pattern? What are the different bits? The 
base value and the operation? So we can write a tree folding
routine as

~~~~~{.haskell}
treeFold op b (Leaf x)   = b
treeFold op b (Node l r) = (treeFold op b l) 
			   `op` 
			   (treeFold op b r)
~~~~~

Does that work? Well, we can easily check that 

~~~~~{.haskell}
size   = treeFold (+) 1
height = treeFold max 0
~~~~~

But what about `toList` ? Urgh. Does. Not. Work. We painted ourselves into
a corner. For `size` and `height` the base value is a constant, but for `toList` 
the base value *depends on* the actual datum at the leaf! Thus, we need
*two* operations: one to combine the results of the left and right
subtrees, and another to give us the value at a leaf. In other words, the
base `b` needs to be a *function* that takes as input the leaf value and
returns as output the result of folding over the leaf.

> treeFold op b (Leaf x)   = b x
> treeFold op b (Node l r) = (treeFold op b l) 
>                            `op` 
>                            (treeFold op b r)

Now we can write the first `height` and `size` as

> height = treeFold max (const 0)
> size   = treeFold (+) (const 1)

where `const 0` and `const 1` are the respective base functions that
ignore the leaf value and just always return `0` and `1` respectively.
Can you guess the definition of `const` ?

What about the problematic `toList` ? Easy enough

> toList = treeFold (++) (\x -> [x]) 

What about the equivalent of `map` for `Tree`s ? We could write
the recursive definition

~~~~~{.haskell}
treeMap :: (a -> b) -> Tree a -> Tree b
treeMap f (Leaf x)   = Leaf (f x)
treeMap f (Node l r) = Node (treeMap f l) (treeMap f r)
~~~~~

but recursion is **HARD TO READ** do we really have to use it ?

**DO IN CLASS**

Sweet! see how we just used the constructors as functions! 
Lets take it out for a spin.

~~~~~{.haskell}
ghci> st1
Node (Leaf "cat") (Leaf "doggerel")
ghci> treeMap length st1
Node (Leaf 3) (Leaf 8)

ghci> st2
Node (Leaf "piglet") (Leaf "hippopotamus")
ghci> treeMap reverse st2
Node (Leaf "telgip") (Leaf "sumatopoppih")
~~~~~