# Recursion & Induction

By Eunice Chen, June 2020

## Types

For each of the following expressions, state its type and value.

fun f (x : int) : int = f x


Type: int -> int
Value: The value is the function itself

val x = []::(150::[])


Type: int list
Value: 

fun f L =
case L of
[] => [[]]
| x::xs => [x] @ []


Type: not well-typed
Value: no value, because it is not well-typed

What is the type and value of x?

fun f (y : int) : int = f y
val x : int = f y


Type: int
Value: No value, because it loops forever

What is the type and value of y?

val x =
let
fun incr (n : int) = n + 1
in
incr
end
val y = incr


Type: No type, since incr is not in scope
Value: No value, since incr is not in scope

## Induction

Given the following function:

(* length: int list -> int *)
fun length [] = 0
| length x::xs = 1 + length(xs)

(* append: int list * int list -> int list *)
fun append ([], L) = L
| append (x::xs, L) = x :: append(xs, L)


Prove that for all L1, L2 : int list, length(append(L1, L2)) = length(L1) + length(L2).

Proof: By structural induction on L1.

BC: L1 = []

length(append(L1, L2))
= length(append([], L2))
= length(L2) [Clause 1 of append]

length(L1) + length(L2)
= length([]) + length(L2)
= 0 + length(L2) [Clause 1 of length]
= length(L2) [math and totality of length]

IS: L1 = x::xs for some x : int, xs : int list

IH: length(append (xs, L2)) = length(xs) + length(L2)

WTS: length(append (x::xs, L2)) = length(x::xs) + length(L2)

length(append (L1, L2))
= length(append (x::xs, L2)
= length (x::append(xs, L2)) [Clause 2 of append]
= 1 + length(append(xs, L2)) [Clause 2 of length, totality of append]
= 1 + length(xs) + length(L2) [IH]

length(L1) + length(L2)
= length(x::xs) + length(L2)
= 1 + length(xs) + length(L2) [Clause 2 of length]

By structural induction, we have shown that for all L1, L2 : int list, length(append(L1, L2)) = length(L1) + length(L2).