# fun with generators

link graphics

procedure main()    
	Window := WOpen("size=500,270") | stop("*** cannot open window")
	WWrite(Window,"              Fun With Generators!")

# using the every expression with the 'to' generator
	WWrites(Window,"\nevery: Enter a string: ")	# 'every' executes the line until it fails
							# by backtracking the expression
	str := WRead(Window)				# 0 to *str generates the numbers between
	every i:= 1 to *str do				# one and the length of the string str
		WWrites(Window,str[i])			# incrimenting by 1 each time it is called
							# then executing the WWrites expression

	WWrites(Window,"\nto-by: Enter a string: ")	# this time the to-by generator is used
	str := WRead(Window)				# and it can incriment in the negative
	every WWrites(Window,str[*str to 1 by -1])	# direction granted that str > 0

	WWrites(Window,"\n!str: Enter a string: ")	# the element generator !e
	str := WRead(Window)				# for strings generates one character
	every WWrites(Window,!str)			# substrings eqivalent to x[1 to *x]


# other ways to induce backtracking other than 'every'	# the &fail expression always fails
	WWrites(Window,"\n&fail: Enter a string: ")	# causing the system to backtrack
	str := WRead(Window)				# and see if it can generate a value
	WWrites(Window,str[1 to *str]) + &fail		# that will allow it to succeed.

# arithmatic generators
	WWrites(Window,"\nEnter a number: ")
	num1 := WRead(Window)
	WWrites(Window,"Enter another number: ")
	num2 := WRead(Window)
	num1 := num1 < num2				# relational operator '<' can be used
							# to generate the greater of the two
							# values.  it returns num2 if true
							# and fails if false exiting the expression
	WWrites(Window,"num1:=",num1,"<",num2," returns: ",num1,"\n")

# conjuction '&' with generators 			
	every i:=1 to num1 & j:=1 to num2 & writes(Window,i+j)
							# & evaluates each generator in sequence
							# and ends when all the generators fails
	WWrites(Window,"\n")

# Alternation e1|e2
	every i:= 1|2|3|4 do WWrites(Window,i)		
							# | or evaluates for each case 
							# before failing
	WWrites(Window,"\n")
# Sequence generation seq(...)
	WWrites(Window,seq(1,10))			# seq works like 'to' except it can 
							# generate infinite numbers by typing
							# seq() or seq(i) which starts at
							# zero or i and counts up
	WRead(Window)
end