1 files changed, 129 insertions, 0 deletions
diff --git a/src/func/helpers.rs b/src/func/helpers.rs
new file mode 100644
index 00000000..d562284f
--- /dev/null
+++ b/src/func/helpers.rs
@@ -0,0 +1,129 @@
+use super::prelude::*;
+use std::iter::Peekable;
+use std::slice::Iter;
+
+/// Implement the function trait more concisely.
+#[macro_export]
+macro_rules! function {
+    (data: $ident:ident, $($tts:tt)*) => {
+        impl Function for $ident {
+            function!(@parse $ident, $($tts)*);
+        }
+    };
+
+    (@parse $ident:ident, parse: plain, $($tts:tt)*) => {
+        fn parse(header: &FuncHeader, body: Option<&str>, _: ParseContext)
+            -> ParseResult<Self> where Self: Sized
+        {
+            Arguments::new(header).done()?;
+            if body.is_some() {
+                err!("expected no body");
+            }
+            Ok($ident)
+        }
+        function!(@layout $($tts)*);
+    };
+
+    (
+        @parse $ident:ident,
+        parse($args:ident, $body:ident, $ctx:ident)
+        $block:block
+        $($tts:tt)*
+    ) => {
+        fn parse(header: &FuncHeader, body: Option<&str>, ctx: ParseContext)
+            -> ParseResult<Self> where Self: Sized
+        {
+            #[allow(unused_mut)] let mut $args = Arguments::new(header);
+            let $body = body;
+            let $ctx = ctx;
+            $block
+        }
+        function!(@layout $($tts)*);
+    };
+
+    (@layout layout($this:pat, $ctx:pat) $block:block) => {
+        fn layout(&self, ctx: LayoutContext) -> LayoutResult<CommandList> {
+            let $ctx = ctx;
+            let $this = self;
+            $block
+        }
+    };
+}
+
+/// Parse the body of a function.
+/// - If the function does not expect a body, use `forbidden`.
+/// - If the function can have a body, use `optional`.
+/// - If the function must have a body, use `required`.
+#[macro_export]
+macro_rules! parse {
+    (forbidden: $body:expr) => {
+        if $body.is_some() {
+            err!("unexpected body");
+        }
+    };
+
+    (optional: $body:expr, $ctx:expr) => {
+        if let Some(body) = $body {
+            Some($crate::parsing::parse(body, $ctx)?)
+        } else {
+            None
+        }
+    };
+
+    (required: $body:expr, $ctx:expr) => {
+        if let Some(body) = $body {
+            $crate::parsing::parse(body, $ctx)?
+        } else {
+            err!("expected body");
+        }
+    }
+}
+
+/// Return a formatted parsing error.
+#[macro_export]
+macro_rules! err {
+    ($($tts:tt)*) => {
+        return Err(ParseError::new(format!($($tts)*)));
+    };
+}
+
+/// Convenient interface for parsing function arguments.
+pub struct Arguments<'a> {
+    args: Peekable<Iter<'a, Expression>>,
+}
+
+impl<'a> Arguments<'a> {
+    pub fn new(header: &'a FuncHeader) -> Arguments<'a> {
+        Arguments {
+            args: header.args.iter().peekable()
+        }
+    }
+
+    pub fn get_expr(&mut self) -> ParseResult<&'a Expression> {
+        self.args.next()
+            .ok_or_else(|| ParseError::new("expected expression"))
+    }
+
+    pub fn get_ident(&mut self) -> ParseResult<&'a str> {
+        match self.get_expr()? {
+            Expression::Ident(s) => Ok(s.as_str()),
+            _ => Err(ParseError::new("expected identifier")),
+        }
+    }
+
+    pub fn get_ident_if_present(&mut self) -> ParseResult<Option<&'a str>> {
+        if self.args.peek().is_some() {
+            self.get_ident().map(|s| Some(s))
+        } else {
+            Ok(None)
+        }
+    }
+
+    pub fn done(&mut self) -> ParseResult<()> {
+        if self.args.peek().is_none() {
+            Ok(())
+        } else {
+            Err(ParseError::new("unexpected argument"))
+        }
+    }
+}