A positive integer is perfect if it equals the sum of all of its factors, excluding the

number itself. Use a list comprehension and the following function factors:

factors :: Int $->[$Int$]$

factors n $=[$x $|$ x $<-[1..$n$],$ n $$mod$$ x $==0]$

to define a function perfects :: Int $->[$Int$]$ that returns the list of all perfect numbers up to

a given limit$.$ For example $($in GHCi$)$:

$>$ perfects $500$

$[6,28,496]$

Hint: define an auxiliary function isperfect :: Int $->$ Bool which returns True if the given Int

is perfect, and False otherwise. Then use isperfect as a guard in a list comprehension to filter

out all of the non$-$perfect integers, i$.$e$.,$ to keep only the perfect integers.