NB. Definitions from Introduction Chapter

t1 =: 5.52

plus =: dyad define
x. + y.
)

NB. alternately
plus =: dyad def 'x. + y.'

square =: monad define
y. * y.
)

maximum =: dyad define
if. x. > y.
  do. x.
  else. y.
end.
)

maximum =: monad define
('x' ; 'y') =. y.
if. x > y
  do. x
  else. y
end.
)

l1 =: 1 2 10 _9 3
l2 =: 'Hi there'

l3 =: (box i. 5) , box i.2

list_sum =: monad define
if. 0 = tally y.
  do. 0
  else. (first y.) + list_sum rest y.
end.
)

traced_list_sum =: monad define
entering y.
if. 0 = tally y.
  do. leaving 0
  else. leaving (first y.) + traced_list_sum rest y.
end.
)

factorial =: monad define
if. 0 = y.
  do. 1
  else. y. * factorial y. - 1
end.
)

traced_factorial =: monad define
entering y.
if. 0 = y.
  do. leaving 1
  else. leaving y. * traced_factorial y. - 1
end.
)

select_positive =: monad define
if. 0 = tally y.
  do. ''
  else.  if. 0 <: first y.
           do. (first y.) append select_positive rest y.
           else. select_positive rest y.
         end.
end.
)

traced_select_positive =: monad define
entering y.
if. 0 = tally y.
  do. leaving ''
  else.  if. 0 <: first y.
           do. leaving (first y.) append traced_select_positive rest y.
           else. leaving traced_select_positive rest y.
         end.
end.
)