Ch6 Discussion 01

Partial Evaluation

P1

Given

f x y = 2*x+3*y
moo = f

What kind of thing is moo? A number? A function? An error?

moo 10 1 == f 10 1
         == 23

P2

Continuing:

oom = f 100 10

What kind of thing is oom? A number? A function? An error?

P3

Continuing:

oof = f 10

What kind of thing is oof? A number? A function? An error?

P4

What if you want to fill in y not x?

f x y = 2*x+3*y

Options:

1. Named function: g x = f x 1
2. Unnamed function (“lambda”): \x -> f x 1
3. Flip: (flip f) 1

What’s flip? It exchanges two inputs, like this:

flip f x y = f y x

Map

List comprehensions apply a function to everything in a list:

[ f x | x <- xs ]

The map function does the same thing:

map f xs

Exercises

Try using some of the methods above (under P4 and map) to do these two exercises several different ways.

1. Given input [3,1,8] produce the output ["abc", "a", "abcdefgh"]. That is, give the first num letters of the alphabet.

2. Input: a list of points. Output: the greatest distance any of those points is from (2,7).