Add Modifying A String With blit_string as a reason 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/).
 
-_729 TILs and counting..._
+_730 TILs and counting..._
 
 ---
 
@@ -534,6 +534,7 @@ _729 TILs and counting..._
 - [Inline Component Styles With Reason React](reason/inline-component-styles-with-reason-react.md)
 - [Is This A Directory Or A File?](reason/is-this-a-directory-or-a-file.md)
 - [Making Things Mutable](reason/making-things-mutable.md)
+- [Modifying A String With blit_string](reason/modifying-a-string-with-blit-string.md)
 - [Multi-Argument Functions As Syntactic Sugar](reason/multi-argument-functions-as-syntactic-sugar.md)
 - [Pattern Match On Exceptions](reason/pattern-match-on-exceptions.md)
 - [Quickly Bootstrap A React App Using Reason](reason/quickly-bootstrap-a-react-app-using-reason.md)

reason/modifying-a-string-with-blit-string.md

@@ -0,0 +1,25 @@
+# Modifying A String With blit_string
+
+[ReasonML's `Bytes` module](https://reasonml.github.io/api/Bytes.html) has a
+function called
+[`blit_string`](https://reasonml.github.io/api/Bytes.html#VALblit_string).
+This function allows you to copy portions of a string into a destination
+byte sequence. It is a fairly low-level operation, so you have to provide a
+source string and provide an offset of that source string to start copying
+from. You then have to provide a properly sized byte sequence as well as the
+destination's starting offset and length of bytes to be copied.
+
+Here is an example of how we can use `blit_string` to create a copy of the
+string with the first character removed.
+
+```reason
+let remove_first_char = (str: string): string => {
+  let copy_len = String.length(str) - 1;
+  let dst = Bytes.create(copy_len);
+  Bytes.blit_string(str, 1, dst, 0, copy_len);
+  Bytes.to_string(dst);
+};
+```
+
+Notice that once the byte sequence has been copied over, we then need to
+convert it back into a string.