add function to convert string to an unescaped string

fcore/escape-string.sml

@@ -0,0 +1,63 @@
+structure EscapeString =
+struct
+  (* The specific escape sequences handled are in the 
+   * `rewriteAcc` function below.
+   * These escape sequences were decided on because
+   * they are the same escape sequences recognised in Standard ML.
+   * A reference is available here: 
+   * https://smlfamily.github.io/Basis/char.html#SIG:CHAR.fromCString:VAL
+   *
+   * However, thre omissions have been made:
+   * - \ddd denoting an integer in the range 0 - 255
+   * - \uxxxx denoting the character whose code is the integer xxxx
+   * - \f f\ denoting a sequence of characters to ignore
+   *
+   * In the first two cases, it is easier to type the character directly.
+   * In the third case, there is little use for it.
+   * *)
+  fun rewriteAcc (#"\\", acc) =
+        (case acc of
+           #"a" :: tl => #"\a" :: tl
+         | #"b" :: tl => #"\b" :: tl
+         | #"t" :: tl => #"\t" :: tl
+         | #"n" :: tl => #"\n" :: tl
+         | #"v" :: tl => #"\v" :: tl
+         | #"f" :: tl => #"\f" :: tl
+         | #"r" :: tl => #"\r" :: tl
+         | #"?" :: tl => #"?" :: tl
+         | #"\\" :: tl => #"\\" :: tl
+         | #"\"" :: tl => #"\"" :: tl
+         | #"'" :: tl => #"'" :: tl
+         | #"^" :: hd :: tl =>
+             (* handle control characters *)
+             let
+               val code = Char.ord hd
+             in
+               if code >= 64 andalso code <= 95 then
+                 let val chr = Char.chr (code - 64)
+                 in chr :: tl
+                 end
+               else
+                 (* invalid escape sequence: leave alone *)
+                 #"\\" :: acc
+             end
+         | _ =>
+             (* when there is no valid escape sequence, 
+              * just leave slash in output *)
+             #"\\" :: acc)
+    | rewriteAcc (chr, acc) = chr :: acc
+
+  fun help (pos, str, acc) =
+    if pos < 0 then
+      String.implode acc
+    else
+      let
+        val chr = String.sub (str, pos)
+        val acc = rewriteAcc (chr, acc)
+      in
+        help (pos - 1, str, acc)
+      end
+
+  fun unescape str =
+    help (String.size str - 1, str, [])
+end