If you rewrite [uri_encode] so that it doesn't use the accumulator [acc]
anymore, but rather does the natural concatenations with recursive
calls, then you should see very efficient imperative code come out of
the Ur/Web compiler, when the result of [uri_encode] is dropped directly
into page output.
On 04/14/2016 07:35 AM, Artyom Shalkhakov wrote:
The code is as follows:
fun
uri_encode (s:string):string = let
fun
tohexstr i = let
val low = i % 16
val high = i / 16
fun hexdigit i =
case i of
0 => "0" | 1 => "1" | 2 => "2" | 3 => "3" | 4 => "4" | 5 => "5" | 6 => "6" | 7 => "7" | 8
=> "8" | 9 => "9"
| 10 => "A" | 11 => "B" | 12 => "C" | 13 => "D" | 14 => "E" | 15 => "F"
| _ => error <xml>tohexstr: invalid digit {[i]}</xml>
in
hexdigit high ^ hexdigit low
end
fun
aux i n s acc =
if i < n then let
val c = strsub s i
in
(* NOTE: strcat seems to be QUITE inefficient here *)
if isalnum c || Option.isSome (String.index ";,/?:@&=+$#" c) then
aux (i+1) n s (strcat acc (str1 c))
else
aux (i+1) n s (strcat acc (strcat "%" (tohexstr (ord c))))
end
else acc
val res = aux 0 (strlen s) s ""
in
res
end
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