Add Stream A File Line By Line as a reason til

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/).
 
-_692 TILs and counting..._
+_693 TILs and counting..._
 
 ---
 
@@ -516,6 +516,7 @@ _692 TILs and counting..._
 - [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)
 - [Seeding And Generating Random Integers](reason/seeding-and-generating-random-integers.md)
+- [Stream A File Line By Line](reason/stream-a-file-line-by-line.md)
 - [String Interpolation With Integers And Sprintf](reason/string-interpolation-with-integers-and-sprintf.md)
 - [String Interpolation With Quoted Strings](reason/string-interpolation-with-quoted-strings.md)
 

reason/stream-a-file-line-by-line.md

@@ -0,0 +1,32 @@
+# Stream A File Line By Line
+
+We can use the `Stream` module in [ReasonML](https://reasonml.github.io/en)
+to read a file getting each line on demand. Doing this requires two key
+insights. First, we can open a file as an _input channel_. Second, we can
+turn an input channel into a _stream_ using `Stream.from`.
+
+```reason
+let file_in_channel = Pervasives.open_in('file.txt');
+
+let file_stream =
+  Stream.from(_i => {
+    switch(Pervasives.input_line(file_in_channel)) {
+    | line => Some(line)
+    | exception(End_of_file) => None
+    };
+  });
+
+file_stream |> Stream.iter(line => do_something(line));
+```
+
+The `Pervasives` module (which is open by default and is only prefixed above
+so as to be explicit) allow us to open the named file as an input channel
+with `open_in`. It also allows us to read lines off that file with
+`input_line`. We use `Stream.from` to create a custom stream that consumes
+the input channel line by line using `input_line`. We either get _some_ line
+or we hit the end of the file. Lastly, we can do whatever we want with the
+stream, such as iterate over it.
+
+See the docs for
+[`Pervasives`](https://reasonml.github.io/api/Pervasives.html) and
+[`Stream`](https://reasonml.github.io/api/Stream.html) for more details.