trap.sml

fun trap(a, b, n, F) = 
    if n <= 0 orelse b-a <= 0.0 then 0.0
    else
	let
	    val delta = (b-a)/real(n)
	in
	    delta * (F(a) + F(a+delta))/2.0 +
		     trap(a+delta, b, n-1, F)
	end;