Add host-structured imports

crates/decodal-core/examples/structured_import.rs

@@ -0,0 +1,87 @@
+use decodal::{
+    Data, Diagnostic, DiagnosticKind, Engine, HostValue, ImportLoader, LoadedImport, Span,
+};
+
+const POST: &str = r#"---
+title: Hello
+draft: false
+---
+# Hello
+
+This body stays as Markdown.
+"#;
+
+struct ContentLoader;
+
+impl ImportLoader for ContentLoader {
+    fn load(
+        &mut self,
+        _current_key: Option<&str>,
+        specifier: &str,
+    ) -> decodal::Result<LoadedImport> {
+        if specifier != "./post.md" {
+            return Err(Diagnostic::new(
+                DiagnosticKind::Import,
+                Span::default(),
+                format!("unknown content import `{specifier}`"),
+            ));
+        }
+        Ok(LoadedImport::value(
+            "content/post.md",
+            parse_markdown(POST)?,
+        ))
+    }
+}
+
+fn parse_markdown(source: &str) -> decodal::Result<HostValue> {
+    let source = source.strip_prefix("---\n").ok_or_else(frontmatter_error)?;
+    let (frontmatter, body) = source.split_once("\n---\n").ok_or_else(frontmatter_error)?;
+    let mut fields = Vec::new();
+    for line in frontmatter.lines() {
+        let (name, value) = line.split_once(':').ok_or_else(frontmatter_error)?;
+        let value = match value.trim() {
+            "true" => HostValue::bool(true),
+            "false" => HostValue::bool(false),
+            value => HostValue::string(value),
+        };
+        fields.push((name.trim(), value));
+    }
+    Ok(HostValue::object([
+        ("frontmatter", HostValue::object(fields)),
+        ("body", HostValue::string(body)),
+    ]))
+}
+
+fn frontmatter_error() -> Diagnostic {
+    Diagnostic::new(
+        DiagnosticKind::Import,
+        Span::default(),
+        "invalid Markdown frontmatter",
+    )
+}
+
+fn main() -> decodal::Result<()> {
+    let mut engine = Engine::new(ContentLoader);
+    let module = engine.add_root_source(
+        "main.dcdl",
+        "main.dcdl",
+        r#"
+        let post = import "./post.md";
+        in {
+            title = post.frontmatter.title;
+            draft = post.frontmatter.draft;
+            body = post.body;
+        }
+        "#,
+    )?;
+    let value = engine.eval_module(module)?;
+    let data = engine.materialize(&value)?;
+
+    let Data::Object(fields) = data else {
+        panic!("expected imported Markdown to produce an object")
+    };
+    assert_eq!(fields[0].value, Data::String(String::from("Hello")));
+    assert_eq!(fields[1].value, Data::Bool(false));
+    assert!(matches!(fields[2].value, Data::String(_)));
+    Ok(())
+}