Add Value type and replace dyn-nodes with call-exprs 🏗

- In addition to syntax trees there are now `Value`s, which syntax trees can be evaluated into (e.g. the tree is `5+5` and the value is `10`)
- Parsing is completely pure, function calls are not parsed into nodes, but into simple call expressions, which are resolved later
- Functions aren't dynamic nodes anymore, but simply functions which receive their arguments as a table and the layouting context
- Functions may return any `Value`
- Layouting is powered by functions which return the new `Commands` value, which informs the layouting engine what to do
- When a function returns a non-`Commands` value, the layouter simply dumps the value into the document in monospace

1 files changed, 37 insertions, 0 deletions

diff --git a/src/prelude.rs b/src/prelude.rs
new file mode 100644
index 00000000..214b00c8
--- /dev/null
+++ b/src/prelude.rs
@@ -0,0 +1,37 @@
+//! A prelude for building custom functions.
+
+pub use crate::compute::value::*;
+pub use crate::layout::prelude::*;
+pub use crate::layout::Commands;
+pub use crate::layout::Command::{self, *};
+pub use crate::style::*;
+pub use crate::syntax::parsing::parse;
+pub use crate::syntax::span::{Pos, Span, SpanVec, Spanned};
+pub use crate::syntax::tree::*;
+pub use crate::{Pass, Feedback};
+pub use super::*;
+
+/// Extra methods on `Option`s used for function argument parsing.
+pub trait OptionExt<T>: Sized {
+    /// Call `f` with `val` if this is `Some(val)`.
+    fn with(self, f: impl FnOnce(T));
+
+    /// Report an error about a missing argument with the given name and span if
+    /// the option is `None`.
+    fn or_missing(self, span: Span, arg: &str, f: &mut Feedback) -> Self;
+}
+
+impl<T> OptionExt<T> for Option<T> {
+    fn with(self, f: impl FnOnce(T)) {
+        if let Some(val) = self {
+            f(val);
+        }
+    }
+
+    fn or_missing(self, span: Span, arg: &str, f: &mut Feedback) -> Self {
+        if self.is_none() {
+            error!(@f, span, "missing argument: {}", arg);
+        }
+        self
+    }
+}