This exercise is about carrying out lazy evaluation and noticing its pros and cons.

First we need to agree on some code:

mysum [] = 0 mysum (x:xt) = x + mysum xt

myor [] = False myor (x:xt) = x || myor xt

"||" is implemented as:

False || c = c True || _ = True

Exercise 1 [3 marks] ----------

Show the lazy evaluation steps of

mysum (1 : 2 : 3 : [])

until you obtain a single number. It is best to add clarifying parentheses: If you have "foo + bar + quux" it is best to write either "(foo + bar) + quux" or "foo + (bar + quux)" to indicate which one you mean.

Exercise 2 [3 marks] ----------

Show the lazy evaluation steps of

myor (True : False : True : [])

until you obtain a single boolean.