document by hand much more of the sprout code

src/extractors.rs

@@ -4,14 +4,25 @@ use anyhow::{Result, bail};
 use serde::{Deserialize, Serialize};
 use std::rc::Rc;
 
+/// The filesystem device match extractor.
 pub mod filesystem_device_match;
 
+/// Declares an extractor configuration.
+/// Extractors allow calculating values at runtime
+/// using built-in sprout modules.
 #[derive(Serialize, Deserialize, Default, Clone)]
 pub struct ExtractorDeclaration {
+    /// The filesystem device match extractor.
+    /// This extractor finds a filesystem using some search criteria and returns
+    /// the device root path that can concatenated with subpaths to access files
+    /// on a particular filesystem.
     #[serde(default, rename = "filesystem-device-match")]
     pub filesystem_device_match: Option<FilesystemDeviceMatchExtractor>,
 }
 
+/// Extracts the value using the specified `extractor` under the provided `context`.
+/// The extractor must return a value, and if a value cannot be determined, an error
+/// should be returned.
 pub fn extract(context: Rc<SproutContext>, extractor: &ExtractorDeclaration) -> Result<String> {
     if let Some(filesystem) = &extractor.filesystem_device_match {
         filesystem_device_match::extract(context, filesystem)