Add Dynamically Create A Printf String Format as a reasonml til

reason/dynamically-create-a-printf-string-format.md

@@ -0,0 +1,34 @@
+# Dynamically Create A Printf String Format
+
+Formatting a string with `Printf` requires defining a format for that
+string.
+
+```reason
+let str = Printf.sprintf("%6s", "dope");
+/* str => "  dope" */
+```
+
+The _format_ is the first argument. At compile-time it is interpreted as a
+`format6` type value.
+
+So, what if you want a dynamically created _format_ value? Simply
+concatenating some strings together won't do it because then the type will
+be `string` and that's not going to compile.
+
+The [`Scanf.format_from_string`](https://reasonml.github.io/api/Scanf.html)
+function can help.
+
+```reason
+let some_num = 6;
+let format_str = "%" ++ string_of_int(some_num) ++ "s";
+let format = Scanf.format_from_string(format_str, "%s");
+
+let str = Printf.sprintf(format, "dope");
+/* str => "  dope" */
+```
+
+We can convert our string that has the appearance of a format into an actual
+`format6` type. To do this, we have to tell `format_from_string` what types
+each of the formats is going to have -- hence the second argument `%s`.
+
+[source](https://twitter.com/rickyvetter/status/1013476235253436417)