Safe `eval` function

2 files changed, 32 insertions, 0 deletions

diff --git a/src/library/mod.rs b/src/library/mod.rs
index 6a30badf..ac0cbb92 100644
--- a/src/library/mod.rs
+++ b/src/library/mod.rs
@@ -73,6 +73,7 @@ pub fn new() -> Scope {
     // Utility.
     std.def_fn("type", utility::type_);
     std.def_fn("assert", utility::assert);
+    std.def_fn("eval", utility::eval);
     std.def_fn("int", utility::int);
     std.def_fn("float", utility::float);
     std.def_fn("abs", utility::abs);
diff --git a/src/library/utility/mod.rs b/src/library/utility/mod.rs
index 13220242..355315e4 100644
--- a/src/library/utility/mod.rs
+++ b/src/library/utility/mod.rs
@@ -8,7 +8,11 @@ pub use color::*;
 pub use math::*;
 pub use string::*;
 
+use std::mem;
+
+use crate::eval::{Eval, Scopes};
 use crate::library::prelude::*;
+use crate::source::SourceFile;
 
 /// The name of a value's type.
 pub fn type_(_: &mut Context, args: &mut Args) -> TypResult<Value> {
@@ -23,3 +27,30 @@ pub fn assert(_: &mut Context, args: &mut Args) -> TypResult<Value> {
     }
     Ok(Value::None)
 }
+
+/// Evaluate a string as Typst markup.
+pub fn eval(ctx: &mut Context, args: &mut Args) -> TypResult<Value> {
+    let Spanned { v: src, span } = args.expect::<Spanned<String>>("source")?;
+
+    // Parse the source and set a synthetic span for all nodes.
+    let mut source = SourceFile::detached(src);
+    source.synthesize(span);
+    let ast = source.ast()?;
+
+    // Save the old context, then detach it.
+    let prev_flow = ctx.flow.take();
+    let prev_route = mem::take(&mut ctx.route);
+
+    // Evaluate the source.
+    let std = ctx.std.clone();
+    let mut scp = Scopes::new(Some(&std));
+    let result = ast.eval(ctx, &mut scp);
+
+    // Restore the old context and handle control flow.
+    ctx.route = prev_route;
+    if let Some(flow) = mem::replace(&mut ctx.flow, prev_flow) {
+        return Err(flow.forbidden());
+    }
+
+    Ok(Value::Content(result?))
+}