Separate syntax set and theme set

This commit separates the handling of syntax sets and theme sets. It
also changes the way how new syntax definitions are loaded from `bat`'s
configuration folder. New syntax definitions are now loaded *in
addition* to the ones that are stored in the `bat` binary by default.

This fixes #172

2 files changed, 62 insertions, 44 deletions

diff --git a/README.md b/README.md
index 010962d4..865d25b5 100644
--- a/README.md
+++ b/README.md
@@ -140,50 +140,33 @@ You can use the `--style` option to control the appearance of `bat`s output.
 You can use `--style=numbers,changes`, for example, to show only Git changes
 and line numbers but no grid and no file header.
 
-### Add new syntaxes and highlighting themes
+### Adding new syntaxes / language definitions
 
 `bat` uses the excellent [`syntect`](https://github.com/trishume/syntect/)
 library for syntax highlighting. `syntect` can read any
 [Sublime Text `.sublime-syntax` file](https://www.sublimetext.com/docs/3/syntax.html)
-and theme.
+and theme. To add new syntax definitions, do the following.
 
-To build your own language-set and theme, follow these steps:
-
-Create a folder with a syntax highlighting theme:
+Create a folder with syntax definition files:
 
 ```bash
 BAT_CONFIG_DIR="$(bat cache --config-dir)"
 
-mkdir -p "$BAT_CONFIG_DIR/themes"
-cd "$BAT_CONFIG_DIR/themes"
-
-# Download a theme, for example:
-git clone https://github.com/greggb/sublime-snazzy
-
-# Create a link for the default theme
-ln -sf "sublime-snazzy/Sublime Snazzy.tmTheme" Default.tmTheme
-```
-
-Create a folder with language definition files:
-
-```bash
 mkdir -p "$BAT_CONFIG_DIR/syntaxes"
 cd "$BAT_CONFIG_DIR/syntaxes"
 
-# Download some language definition files, for example:
-git clone https://github.com/sublimehq/Packages
-git clone https://github.com/danro/LESS-sublime
+# Put new '.sublime-syntax' language definition files
+# in this folder (or its subdirectories), for example:
+git clone https://github.com/tellnobody1/sublime-purescript-syntax
 ```
 
-Finally, use the following command to parse all these files into a binary
-cache:
+Now use the following command to parse these files into a binary cache:
 
 ```bash
 bat cache --init
 ```
 
-Use `bat --list-languages` and `bat --list-themes` to check if all languages and themes are
-available.
+Finally, use `bat --list-languages` to check if the new languages are available.
 
 If you ever want to go back to the default settings, call:
 
@@ -191,6 +174,32 @@ If you ever want to go back to the default settings, call:
 bat cache --clear
 ```
 
+### Adding new themes
+
+This works very similar to how we add new syntax definitions.
+
+First, create a folder with the new syntax highlighting themes:
+```bash
+BAT_CONFIG_DIR="$(bat cache --config-dir)"
+
+mkdir -p "$BAT_CONFIG_DIR/themes"
+cd "$BAT_CONFIG_DIR/themes"
+
+# Download a theme in '.tmTheme' format, for example:
+git clone https://github.com/greggb/sublime-snazzy
+
+# Create a link to specify the new default theme
+ln -sf "sublime-snazzy/Sublime Snazzy.tmTheme" Default.tmTheme
+
+# Update the binary cache
+bat cache --init
+```
+
+Finally, use `bat --list-themes` to check if the new themes are available.
+
+**Note:** Unlike for syntax definitions, adding custom themes currently *removes all default
+themes*. If you want to go back to the default themes, call `bat cache --clear`.
+
 ### Using a different pager
 
 `bat` uses the pager that is specified in the `PAGER` environment variable. If this variable is not
diff --git a/src/assets.rs b/src/assets.rs
index 7cddd186..c497cfa4 100644
--- a/src/assets.rs
+++ b/src/assets.rs
@@ -28,28 +28,32 @@ impl HighlightingAssets {
     pub fn from_files(dir: Option<&Path>) -> Result<Self> {
         let source_dir = dir.unwrap_or_else(|| PROJECT_DIRS.config_dir());
 
+        let mut assets = Self::from_binary_unlinked();
+
         let theme_dir = source_dir.join("themes");
-        let theme_set = ThemeSet::load_from_folder(&theme_dir).chain_err(|| {
-            format!(
-                "Could not load themes from '{}'",
+
+        if let Ok(theme_set) = ThemeSet::load_from_folder(&theme_dir) {
+            // TODO: If syntect would support this, it would be great to
+            // load the new themes in addition to the ones in the binary.
+            assets.theme_set = theme_set;
+        } else {
+            println!(
+                "No themes were found in '{}', using the default set",
                 theme_dir.to_string_lossy()
-            )
-        })?;
-        let mut syntax_set = SyntaxSet::new();
+            );
+        }
+
         let syntax_dir = source_dir.join("syntaxes");
-        if !syntax_dir.exists() {
-            return Err(format!(
-                "Could not load syntaxes from '{}'",
+        if syntax_dir.exists() {
+            assets.syntax_set.load_syntaxes(syntax_dir, true)?;
+        } else {
+            println!(
+                "No syntaxes were found in '{}', using the default set.",
                 syntax_dir.to_string_lossy()
-            ).into());
+            );
         }
-        syntax_set.load_syntaxes(syntax_dir, true)?;
-        syntax_set.load_plain_text_syntax();
 
-        Ok(HighlightingAssets {
-            syntax_set,
-            theme_set,
-        })
+        Ok(assets)
     }
 
     fn from_cache() -> Result<Self> {
@@ -79,9 +83,8 @@ impl HighlightingAssets {
         })
     }
 
-    fn from_binary() -> Self {
-        let mut syntax_set: SyntaxSet = from_binary(include_bytes!("../assets/syntaxes.bin"));
-        syntax_set.link_syntaxes();
+    fn from_binary_unlinked() -> Self {
+        let syntax_set: SyntaxSet = from_binary(include_bytes!("../assets/syntaxes.bin"));
         let theme_set: ThemeSet = from_binary(include_bytes!("../assets/themes.bin"));
 
         HighlightingAssets {
@@ -90,6 +93,12 @@ impl HighlightingAssets {
         }
     }
 
+    fn from_binary() -> Self {
+        let mut assets = Self::from_binary_unlinked();
+        assets.syntax_set.link_syntaxes();
+        assets
+    }
+
     pub fn save(&self, dir: Option<&Path>) -> Result<()> {
         let target_dir = dir.unwrap_or_else(|| PROJECT_DIRS.cache_dir());
         let _ = fs::create_dir(target_dir);