Destructive Assignment in SML

I won't lie; this is for a homework assignment, but I'm not asking it
to be done for me; I just want to know how to accomplish destructive
assingment in ML.  The problem is to construct a function with
memoization, so here's what I have:

local
    val memory = []
in
  fun fastF 0  = 1 | 
    fastF ~1 = 0 | 
    fastF ~2 = 0 | 
    fastF ~3 = 0 | 
    fastF m  =   
        let 
            val result = find (fn (key, value) => key = m) memory
        in  
            if isSome result then
                #2(result)
            else
              let 
                val result = fastF(m-1) + fastF(m-2) + fastF(m-3) -
                    fastF(m-4) 
	      in  
                ((val memory = ((m, result) :: memory)); result)
              end 
        end 

Unfortunately, (val memory = ...) doesn't accomplish what I need; it
causes a syntax error.  Could somone shed some light on how I could
assign to memory?  Thanks!