Add Dynamically Create A Printf String Format as a reasonml til

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_684 TILs and counting..._
+_685 TILs and counting..._
 
 ---
 
@@ -503,6 +503,7 @@ _684 TILs and counting..._
 
 - [Break Out Of A While Loop](reason/break-out-of-a-while-loop.md)
 - [Defining Variants With Constructor Arguments](reason/defining-variants-with-constructor-arguments.md)
+- [Dynamically Create A Printf String Format](reason/dynamically-create-a-printf-string-format.md)
 - [Exhaustive Pattern Matching Of List Variants](reason/exhaustive-pattern-matching-of-list-variants.md)
 - [Helping The Compiler Help Us With Variants](reason/helping-the-compiler-help-us-with-variants.md)
 - [Inline Component Styles With Reason React](reason/inline-component-styles-with-reason-react.md)

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)