everybody

I am trying to implement simple algorithm "**Shunting yard**" (**to parse infix to postfix notation**) using **SML** (**Standart ML**)

```
fun parseToRPN(input:string) =
let
val _input = explode input
val digits = [#"0", #"1", #"2", #"3", #"4", #"5", #"6", #"7", #"8", #"9"]
val stack = []
val output = []
exception Empty
fun member(n:char, nil) = false
| member(n:char, h::t) = if n = h then true else member(n:char, t);
fun parse(nil) = raise Empty
| parse(h::t) = if member(h, digits) then h::parse(t)
else if h = #"(" then ???
in
...
end
```

But I have no idea how to push to stack in SML. Can somebody give some tips! Thank you!