Simple dynamic, scoped function parsing 📥

1 files changed, 93 insertions, 0 deletions

diff --git a/src/func.rs b/src/func.rs
new file mode 100644
index 00000000..cbc2f58b
--- /dev/null
+++ b/src/func.rs
@@ -0,0 +1,93 @@
+//! Dynamic typesetting functions.
+
+use std::any::Any;
+use std::collections::HashMap;
+use std::fmt::Debug;
+use crate::syntax::{Token, FuncHeader, Expression};
+use crate::parsing::ParseError;
+
+
+/// An optional iterator over the tokens of a function body.
+pub type BodyTokens<'a> = Option<Box<dyn Iterator<Item=Token<'a>> + 'a>>;
+
+
+/// Types that act as functions.
+///
+/// These types have to be able to parse tokens into themselves and store the
+/// relevant information from the parsing to do their role in typesetting later.
+///
+/// `FunctionRequirements` is automatically implemented for types which
+/// can be used as functions, that is they fulfill the bounds
+/// `Debug + PartialEq + 'static`.
+pub trait Function: FunctionRequirements + 'static {
+    /// Parse the function.
+    fn parse(header: &FuncHeader, tokens: BodyTokens<'_>, scope: &Scope)
+    -> Result<Self, ParseError> where Self: Sized;
+
+    /// Execute the function and optionally yield a return value.
+    fn typeset(&self, header: &FuncHeader) -> Option<Expression>;
+}
+
+/// A helper trait that describes requirements for types implement [`Function`].
+///
+/// Automatically implemented for all types which fulfill to the bounds
+/// `Debug + PartialEq + 'static`. There should be no need to implement this
+/// manually.
+pub trait FunctionRequirements: Debug {
+    /// Cast self into `Any`.
+    fn help_cast_as_any(&self) -> &dyn Any;
+
+    /// Compare self with another function.
+    fn help_eq(&self, other: &dyn Function) -> bool;
+}
+
+impl<T> FunctionRequirements for T where T: Debug + PartialEq + 'static {
+    fn help_cast_as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn help_eq(&self, other: &dyn Function) -> bool {
+        if let Some(other) = other.help_cast_as_any().downcast_ref::<Self>() {
+            self == other
+        } else {
+            false
+        }
+    }
+}
+
+impl PartialEq for dyn Function {
+    fn eq(&self, other: &dyn Function) -> bool {
+        self.help_eq(other)
+    }
+}
+
+/// A map from identifiers to functions.
+pub struct Scope {
+    parsers: HashMap<String, Box<dyn Fn(&FuncHeader, BodyTokens<'_>, &Scope)
+                                     -> Result<Box<dyn Function>, ParseError>>>,
+}
+
+impl Scope {
+    /// Create a new empty scope.
+    pub fn new() -> Scope {
+        Scope { parsers: HashMap::new() }
+    }
+
+    /// Add a function type to the scope with a given name.
+    pub fn add<F: Function + 'static>(&mut self, name: &str) {
+        self.parsers.insert(
+            name.to_owned(),
+            Box::new(|header, tokens, scope| match F::parse(header, tokens, scope) {
+                Ok(func) => Ok(Box::new(func)),
+                Err(err) => Err(err),
+            })
+        );
+    }
+
+    /// Return the parser with the given name if there is one.
+    pub fn get_parser(&self, name: &str)
+        -> Option<&dyn Fn(&FuncHeader, BodyTokens<'_>, &Scope)
+            -> Result<Box<dyn Function>, ParseError>> {
+        self.parsers.get(name).map(|x| &**x)
+    }
+}