Accept filepaths through the CLI (#12)

src/cli/src/main.rs

@@ -1,18 +1,28 @@
 extern crate clap;
 
-use clap::App;
+use clap::{App, Arg};
 use diagram_base::TransformError;
+use std::fs::File;
 use std::io::{self, Read};
+use std::path::Path;
 use std::process;
 
 const PKG_NAME: &str = env!("CARGO_PKG_NAME");
 const PKG_VERSION: &str = env!("CARGO_PKG_VERSION");
 const PKG_AUTHORS: &str = env!("CARGO_PKG_AUTHORS");
 
 fn read_from_stdin() -> io::Result<String> {
-    let mut buffer = String::new();
     let stdin = io::stdin();
     let mut handle = stdin.lock();
+    let mut buffer = String::new();
+    handle.read_to_string(&mut buffer)?;
+    Ok(buffer)
+}
+
+fn read_from_path(path: &str) -> io::Result<String> {
+    let path = Path::new(path);
+    let mut handle = File::open(&path)?;
+    let mut buffer = String::new();
     handle.read_to_string(&mut buffer)?;
     Ok(buffer)
 }
@@ -23,13 +33,30 @@ fn render(input: &str) -> Result<String, TransformError> {
 }
 
 fn main() -> io::Result<()> {
-    App::new(PKG_NAME)
+    let matches = App::new(PKG_NAME)
         .version(PKG_VERSION)
         .about("Diagrams as code")
         .author(PKG_AUTHORS)
+        .arg(
+            Arg::with_name("PATH")
+                .help("Path to the .diag file to generate diagram for (- for STDIN).")
+                .required(false)
+                .index(1),
+        )
         .get_matches();
 
-    let input = read_from_stdin()?;
+    let input = {
+        let path = matches.value_of("PATH").unwrap_or("-");
+        let (verbose_path, result) = match path {
+            "-" => ("STDIN", read_from_stdin()),
+            path => (path, read_from_path(path)),
+        };
+        result.unwrap_or_else(|e| {
+            eprintln!("{}: {}", verbose_path, e);
+            process::exit(1);
+        })
+    };
+
     let output = render(input.as_str());
     match output {
         Ok(repr) if !repr.is_empty() => println!("{}", repr),