Merge pull request #112 from Nadrieril/improve-parser

Add features to parser macros

4 files changed, 478 insertions, 310 deletions

diff --git a/dhall_generated_parser/src/dhall.pest.visibility b/dhall_generated_parser/src/dhall.pest.visibility
index dcebf45..17c1edc 100644
--- a/dhall_generated_parser/src/dhall.pest.visibility
+++ b/dhall_generated_parser/src/dhall.pest.visibility
@@ -18,7 +18,7 @@
 simple_label
 # quoted_label_char
 quoted_label
-label
+# label
 # nonreserved_label
 # any_label
 double_quote_chunk
@@ -31,13 +31,13 @@ escaped_quote_pair
 escaped_interpolation
 single_quote_char
 single_quote_literal
-interpolation
+# interpolation
 # text_literal
 if_
 # then
 # else_
 # let_
-in_
+# in_
 # as_
 # using
 merge
@@ -49,9 +49,9 @@ toMap
 assert
 # keyword
 builtin
-Optional
+# Optional
 Text
-List
+# List
 Location
 # Bool
 # True
@@ -95,7 +95,7 @@ arrow
 numeric_double_literal
 minus_infinity_literal
 plus_infinity_literal
-double_literal
+# double_literal
 natural_literal
 integer_literal
 identifier
@@ -104,7 +104,7 @@ variable
 # quoted_path_character
 unquoted_path_component
 quoted_path_component
-path_component
+# path_component
 path
 local
 parent_path
@@ -136,7 +136,7 @@ env
 bash_environment_variable
 posix_environment_variable
 posix_environment_variable_character
-import_type
+# import_type
 hash
 import_hashed
 import
@@ -160,7 +160,7 @@ not_equal_expression
 equivalent_expression
 application_expression
 first_application_expression
-import_expression
+# import_expression
 selector_expression
 selector
 labels
diff --git a/dhall_proc_macros/src/make_parser.rs b/dhall_proc_macros/src/make_parser.rs
index 268a639..a17ab61 100644
--- a/dhall_proc_macros/src/make_parser.rs
+++ b/dhall_proc_macros/src/make_parser.rs
@@ -1,70 +1,188 @@
+use std::collections::HashMap;
+use std::iter;
+
 use quote::quote;
-use syn::parse::{ParseStream, Result};
+use syn::parse::{Parse, ParseStream, Result};
 use syn::spanned::Spanned;
 use syn::{
     parse_quote, Error, Expr, FnArg, Ident, ImplItem, ImplItemMethod, ItemImpl,
-    Pat, Token,
+    LitBool, Pat, Token,
 };
 
-fn apply_special_attrs(function: &mut ImplItemMethod) -> Result<()> {
+mod kw {
+    syn::custom_keyword!(shortcut);
+}
+
+struct AliasArgs {
+    target: Ident,
+    is_shortcut: bool,
+}
+
+struct PrecClimbArgs {
+    child_rule: Ident,
+    climber: Expr,
+}
+
+struct AliasSrc {
+    ident: Ident,
+    is_shortcut: bool,
+}
+
+struct ParsedFn<'a> {
+    // Body of the function
+    function: &'a mut ImplItemMethod,
+    // Name of the function.
+    fn_name: Ident,
+    // Name of the first argument of the function, which should be of type `ParseInput`.
+    input_arg: Ident,
+    // List of aliases pointing to this function
+    alias_srcs: Vec<AliasSrc>,
+}
+
+impl Parse for AliasArgs {
+    fn parse(input: ParseStream) -> Result<Self> {
+        let target = input.parse()?;
+        let is_shortcut = if input.peek(Token![,]) {
+            // #[alias(rule, shortcut = true)]
+            let _: Token![,] = input.parse()?;
+            let _: kw::shortcut = input.parse()?;
+            let _: Token![=] = input.parse()?;
+            let b: LitBool = input.parse()?;
+            b.value
+        } else {
+            // #[alias(rule)]
+            false
+        };
+        Ok(AliasArgs {
+            target,
+            is_shortcut,
+        })
+    }
+}
+
+impl Parse for PrecClimbArgs {
+    fn parse(input: ParseStream) -> Result<Self> {
+        let child_rule = input.parse()?;
+        let _: Token![,] = input.parse()?;
+        let climber = input.parse()?;
+        Ok(PrecClimbArgs {
+            child_rule,
+            climber,
+        })
+    }
+}
+
+fn collect_aliases(
+    imp: &mut ItemImpl,
+) -> Result<HashMap<Ident, Vec<AliasSrc>>> {
+    let functions = imp.items.iter_mut().flat_map(|item| match item {
+        ImplItem::Method(m) => Some(m),
+        _ => None,
+    });
+
+    let mut alias_map = HashMap::new();
+    for function in functions {
+        let fn_name = function.sig.ident.clone();
+        let mut alias_attrs = function
+            .attrs
+            .drain_filter(|attr| attr.path.is_ident("alias"))
+            .collect::<Vec<_>>()
+            .into_iter();
+
+        if let Some(attr) = alias_attrs.next() {
+            let args: AliasArgs = attr.parse_args()?;
+            alias_map.entry(args.target).or_insert_with(Vec::new).push(
+                AliasSrc {
+                    ident: fn_name,
+                    is_shortcut: args.is_shortcut,
+                },
+            );
+        }
+        if let Some(attr) = alias_attrs.next() {
+            return Err(Error::new(
+                attr.span(),
+                "expected at most one alias attribute",
+            ));
+        }
+    }
+
+    Ok(alias_map)
+}
+
+fn parse_fn<'a>(
+    function: &'a mut ImplItemMethod,
+    alias_map: &mut HashMap<Ident, Vec<AliasSrc>>,
+) -> Result<ParsedFn<'a>> {
+    let fn_name = function.sig.ident.clone();
+    // Get the name of the first (`input`) function argument
+    let input_arg = function.sig.inputs.first().ok_or_else(|| {
+        Error::new(
+            function.sig.inputs.span(),
+            "a rule function needs an `input` argument",
+        )
+    })?;
+    let input_arg = match &input_arg {
+        FnArg::Receiver(_) => return Err(Error::new(
+            input_arg.span(),
+            "a rule function should not have a `self` argument",
+        )),
+        FnArg::Typed(input_arg) => match &*input_arg.pat{
+            Pat::Ident(ident) => ident.ident.clone(),
+            _ => return Err(Error::new(
+                input_arg.span(),
+                "this argument should be a plain identifier instead of a pattern",
+            )),
+        }
+    };
+
+    let alias_srcs = alias_map.remove(&fn_name).unwrap_or_else(Vec::new);
+
+    Ok(ParsedFn {
+        function,
+        fn_name,
+        input_arg,
+        alias_srcs,
+    })
+}
+
+fn apply_special_attrs(f: &mut ParsedFn, rule_enum: &Ident) -> Result<()> {
+    let function = &mut *f.function;
+    let fn_name = &f.fn_name;
+    let input_arg = &f.input_arg;
+
     *function = parse_quote!(
-        #[allow(non_snake_case, dead_code)]
+        #[allow(non_snake_case)]
         #function
     );
 
-    let recognized_attrs: Vec<_> = function
+    // `prec_climb` attr
+    let prec_climb_attrs: Vec<_> = function
         .attrs
         .drain_filter(|attr| attr.path.is_ident("prec_climb"))
         .collect();
 
-    let name = function.sig.ident.clone();
-
-    if recognized_attrs.is_empty() {
-        // do nothing
-    } else if recognized_attrs.len() > 1 {
+    if prec_climb_attrs.len() > 1 {
         return Err(Error::new(
-            recognized_attrs[1].span(),
-            "expected a single prec_climb attribute",
+            prec_climb_attrs[1].span(),
+            "expected at most one prec_climb attribute",
         ));
+    } else if prec_climb_attrs.is_empty() {
+        // do nothing
     } else {
-        let attr = recognized_attrs.into_iter().next().unwrap();
-        let (child_rule, climber) =
-            attr.parse_args_with(|input: ParseStream| {
-                let child_rule: Ident = input.parse()?;
-                let _: Token![,] = input.parse()?;
-                let climber: Expr = input.parse()?;
-                Ok((child_rule, climber))
-            })?;
-
-        // Get the name of the first (`input`) function argument
-        let first_arg = function.sig.inputs.first().ok_or_else(|| {
-            Error::new(
-                function.sig.inputs.span(),
-                "a prec_climb function needs 4 arguments",
-            )
-        })?;
-        let first_arg = match &first_arg {
-            FnArg::Receiver(_) => return Err(Error::new(
-                first_arg.span(),
-                "a prec_climb function should not have a `self` argument",
-            )),
-            FnArg::Typed(first_arg) =>   match &*first_arg.pat{
-                Pat::Ident(ident) => &ident.ident,
-                _ => return Err(Error::new(
-                    first_arg.span(),
-                    "this argument should be a plain identifier instead of a pattern",
-                )),
-            }
-        };
+        let attr = prec_climb_attrs.into_iter().next().unwrap();
+        let PrecClimbArgs {
+            child_rule,
+            climber,
+        } = attr.parse_args()?;
 
         function.block = parse_quote!({
             #function
 
             #climber.climb(
-                #first_arg.pair.clone().into_inner(),
-                |p| Self::#child_rule(#first_arg.with_pair(p)),
+                #input_arg.pair.clone().into_inner(),
+                |p| Self::#child_rule(#input_arg.with_pair(p)),
                 |l, op, r| {
-                    #name(#first_arg.clone(), l?, op, r?)
+                    #fn_name(#input_arg.clone(), l?, op, r?)
                 },
             )
         });
@@ -81,6 +199,37 @@ fn apply_special_attrs(function: &mut ImplItemMethod) -> Result<()> {
         })?;
     }
 
+    // `alias` attr
+    if !f.alias_srcs.is_empty() {
+        let aliases = f.alias_srcs.iter().map(|src| &src.ident);
+        let block = &function.block;
+        function.block = parse_quote!({
+            let mut #input_arg = #input_arg;
+            // While the current rule allows shortcutting, and there is a single child, and the
+            // child can still be parsed by the current function, then skip to that child.
+            while <Self as PestConsumer>::allows_shortcut(#input_arg.as_rule()) {
+                if let Some(child) = #input_arg.single_child() {
+                    if &<Self as PestConsumer>::rule_alias(child.as_rule())
+                            == stringify!(#fn_name) {
+                        #input_arg = child;
+                        continue;
+                    }
+                }
+                break
+            }
+
+            match #input_arg.as_rule() {
+                #(#rule_enum::#aliases => Self::#aliases(#input_arg),)*
+                #rule_enum::#fn_name => #block,
+                r => unreachable!(
+                    "make_parser: called {} on {:?}",
+                    stringify!(#fn_name),
+                    r
+                )
+            }
+        });
+    }
+
     Ok(())
 }
 
@@ -89,21 +238,95 @@ pub fn make_parser(
     input: proc_macro::TokenStream,
 ) -> Result<proc_macro2::TokenStream> {
     let rule_enum: Ident = syn::parse(attrs)?;
-
     let mut imp: ItemImpl = syn::parse(input)?;
-    imp.items
+
+    let mut alias_map = collect_aliases(&mut imp)?;
+    let rule_alias_branches: Vec<_> = alias_map
+        .iter()
+        .flat_map(|(tgt, srcs)| iter::repeat(tgt).zip(srcs))
+        .map(|(tgt, src)| {
+            let ident = &src.ident;
+            quote!(
+                #rule_enum::#ident => stringify!(#tgt).to_string(),
+            )
+        })
+        .collect();
+    let shortcut_branches: Vec<_> = alias_map
+        .iter()
+        .flat_map(|(_tgt, srcs)| srcs)
+        .map(|AliasSrc { ident, is_shortcut }| {
+            quote!(
+                #rule_enum::#ident => #is_shortcut,
+            )
+        })
+        .collect();
+
+    let fn_map: HashMap<Ident, ParsedFn> = imp
+        .items
         .iter_mut()
-        .map(|item| match item {
-            ImplItem::Method(m) => apply_special_attrs(m),
-            _ => Ok(()),
+        .flat_map(|item| match item {
+            ImplItem::Method(m) => Some(m),
+            _ => None,
+        })
+        .map(|method| {
+            let mut f = parse_fn(method, &mut alias_map)?;
+            apply_special_attrs(&mut f, &rule_enum)?;
+            Ok((f.fn_name.clone(), f))
+        })
+        .collect::<Result<_>>()?;
+
+    // Entries that remain in the alias map don't have a matching method, so we create one.
+    let extra_fns: Vec<_> = alias_map
+        .iter()
+        .map(|(tgt, srcs)| {
+            // Get the signature of one of the functions that has this alias. They should all have
+            // essentially the same signature anyways.
+            let f = fn_map.get(&srcs.first().unwrap().ident).unwrap();
+            let input_arg = f.input_arg.clone();
+            let mut sig = f.function.sig.clone();
+            sig.ident = tgt.clone();
+            let srcs = srcs.iter().map(|src| &src.ident);
+
+            Ok(parse_quote!(
+                #sig {
+                    match #input_arg.as_rule() {
+                        #(#rule_enum::#srcs => Self::#srcs(#input_arg),)*
+                        // We can't match on #rule_enum::#tgt since `tgt` might be an arbitrary
+                        // identifier.
+                        r if &format!("{:?}", r) == stringify!(#tgt) =>
+                            return Err(#input_arg.error(format!(
+                                "make_parser: missing method for rule {}",
+                                stringify!(#tgt),
+                            ))),
+                        r => unreachable!(
+                            "make_parser: called {} on {:?}",
+                            stringify!(#tgt),
+                            r
+                        )
+                    }
+                }
+            ))
         })
-        .collect::<Result<()>>()?;
+        .collect::<Result<_>>()?;
+    imp.items.extend(extra_fns);
 
     let ty = &imp.self_ty;
     let (impl_generics, _, where_clause) = imp.generics.split_for_impl();
     Ok(quote!(
         impl #impl_generics PestConsumer for #ty #where_clause {
-            type RuleEnum = #rule_enum;
+            type Rule = #rule_enum;
+            fn rule_alias(rule: Self::Rule) -> String {
+                match rule {
+                    #(#rule_alias_branches)*
+                    r => format!("{:?}", r),
+                }
+            }
+            fn allows_shortcut(rule: Self::Rule) -> bool {
+                match rule {
+                    #(#shortcut_branches)*
+                    _ => false,
+                }
+            }
         }
 
         #imp
diff --git a/dhall_proc_macros/src/parse_children.rs b/dhall_proc_macros/src/parse_children.rs
index b1d43fc..a35c03f 100644
--- a/dhall_proc_macros/src/parse_children.rs
+++ b/dhall_proc_macros/src/parse_children.rs
@@ -88,9 +88,7 @@ fn make_parser_branch(
     let i_variable_pattern =
         Ident::new("___variable_pattern", Span::call_site());
     let match_pat = branch.pattern.iter().map(|item| match item {
-        Single { rule_name, .. } => {
-            quote!(<<Self as PestConsumer>::RuleEnum>::#rule_name)
-        }
+        Single { rule_name, .. } => quote!(stringify!(#rule_name)),
         Multiple { .. } => quote!(#i_variable_pattern @ ..),
     });
     let match_filter = branch.pattern.iter().map(|item| match item {
@@ -101,7 +99,7 @@ fn make_parser_branch(
                 // https://github.com/rust-lang/rust/issues/59803.
                 let all_match = |slice: &[_]| {
                     slice.iter().all(|r|
-                        r == &<<Self as PestConsumer>::RuleEnum>::#rule_name
+                        *r == stringify!(#rule_name)
                     )
                 };
                 all_match(#i_variable_pattern)
@@ -192,6 +190,11 @@ pub fn parse_children(
             .clone()
             .into_inner()
             .map(|p| p.as_rule())
+            .map(<Self as PestConsumer>::rule_alias)
+            .collect();
+        let #i_children_rules: Vec<&str> = #i_children_rules
+            .iter()
+            .map(String::as_str)
             .collect();
 
         #[allow(unused_mut)]
diff --git a/dhall_syntax/src/parser.rs b/dhall_syntax/src/parser.rs
index c2ba19a..3f961e8 100644
--- a/dhall_syntax/src/parser.rs
+++ b/dhall_syntax/src/parser.rs
@@ -60,17 +60,32 @@ impl<'input> ParseInput<'input, Rule> {
             original_input_str: self.original_input_str.clone(),
         }
     }
+    /// If the contained pair has exactly one child, return a new Self containing it.
+    fn single_child(&self) -> Option<Self> {
+        let mut children = self.pair.clone().into_inner();
+        if let Some(child) = children.next() {
+            if children.next().is_none() {
+                return Some(self.with_pair(child));
+            }
+        }
+        None
+    }
     fn as_span(&self) -> Span {
         Span::make(self.original_input_str.clone(), self.pair.as_span())
     }
     fn as_str(&self) -> &'input str {
         self.pair.as_str()
     }
+    fn as_rule(&self) -> Rule {
+        self.pair.as_rule()
+    }
 }
 
-// Used to retrieve the `Rule` enum associated with the `Self` type in `parse_children`.
+// Used by the macros.
 trait PestConsumer {
-    type RuleEnum: pest::RuleType;
+    type Rule: pest::RuleType;
+    fn rule_alias(rule: Self::Rule) -> String;
+    fn allows_shortcut(rule: Self::Rule) -> bool;
 }
 
 fn debug_pair(pair: Pair<Rule>) -> String {
@@ -231,21 +246,17 @@ struct Parsers;
 
 #[make_parser(Rule)]
 impl Parsers {
-    fn EOI(_: ParseInput<Rule>) -> ParseResult<()> {
+    fn EOI(_input: ParseInput<Rule>) -> ParseResult<()> {
         Ok(())
     }
 
+    #[alias(label)]
     fn simple_label(input: ParseInput<Rule>) -> ParseResult<Label> {
-        Ok(Label::from(input.as_str().trim().to_owned()))
+        Ok(Label::from(input.as_str()))
     }
+    #[alias(label)]
     fn quoted_label(input: ParseInput<Rule>) -> ParseResult<Label> {
-        Ok(Label::from(input.as_str().trim().to_owned()))
-    }
-    fn label(input: ParseInput<Rule>) -> ParseResult<Label> {
-        Ok(parse_children!(input;
-            [simple_label(l)] => l,
-            [quoted_label(l)] => l,
-        ))
+        Ok(Label::from(input.as_str()))
     }
 
     fn double_quote_literal<E: Clone>(
@@ -262,17 +273,15 @@ impl Parsers {
         input: ParseInput<Rule>,
     ) -> ParseResult<ParsedTextContents<E>> {
         Ok(parse_children!(input;
-            [interpolation(e)] => {
+            [expression(e)] => {
                 InterpolatedTextContents::Expr(e)
             },
-            [double_quote_escaped(s)] => {
-                InterpolatedTextContents::Text(s)
-            },
             [double_quote_char(s)] => {
-                InterpolatedTextContents::Text(s.to_owned())
+                InterpolatedTextContents::Text(s)
             },
         ))
     }
+    #[alias(double_quote_char)]
     fn double_quote_escaped(input: ParseInput<Rule>) -> ParseResult<String> {
         Ok(match input.as_str() {
             "\"" => "\"".to_owned(),
@@ -345,8 +354,8 @@ impl Parsers {
     }
     fn double_quote_char<'a>(
         input: ParseInput<'a, Rule>,
-    ) -> ParseResult<&'a str> {
-        Ok(input.as_str())
+    ) -> ParseResult<String> {
+        Ok(input.as_str().to_owned())
     }
 
     fn single_quote_literal<E: Clone>(
@@ -356,6 +365,7 @@ impl Parsers {
             [single_quote_continue(lines)] => {
                 let newline: ParsedText<E> = "\n".to_string().into();
 
+                // Reverse lines and chars in each line
                 let mut lines: Vec<ParsedText<E>> = lines
                     .into_iter()
                     .rev()
@@ -377,47 +387,30 @@ impl Parsers {
     ) -> ParseResult<&'a str> {
         Ok(input.as_str())
     }
-    fn escaped_quote_pair<'a>(_: ParseInput<'a, Rule>) -> ParseResult<&'a str> {
+    #[alias(single_quote_char)]
+    fn escaped_quote_pair<'a>(
+        _input: ParseInput<'a, Rule>,
+    ) -> ParseResult<&'a str> {
         Ok("''")
     }
+    #[alias(single_quote_char)]
     fn escaped_interpolation<'a>(
-        _: ParseInput<'a, Rule>,
+        _input: ParseInput<'a, Rule>,
     ) -> ParseResult<&'a str> {
         Ok("${")
     }
-    fn interpolation<E: Clone>(
-        input: ParseInput<Rule>,
-    ) -> ParseResult<Expr<E>> {
-        Ok(parse_children!(input;
-            [expression(e)] => e
-        ))
-    }
 
     // Returns a vec of lines in reversed order, where each line is also in reversed order.
     fn single_quote_continue<E: Clone>(
         input: ParseInput<Rule>,
     ) -> ParseResult<Vec<Vec<ParsedTextContents<E>>>> {
         Ok(parse_children!(input;
-            [interpolation(c), single_quote_continue(lines)] => {
-                let c = InterpolatedTextContents::Expr(c);
+            [expression(e), single_quote_continue(lines)] => {
+                let c = InterpolatedTextContents::Expr(e);
                 let mut lines = lines;
                 lines.last_mut().unwrap().push(c);
                 lines
             },
-            [escaped_quote_pair(c), single_quote_continue(lines)] => {
-                let mut lines = lines;
-                // TODO: don't allocate for every char
-                let c = InterpolatedTextContents::Text(c.to_owned());
-                lines.last_mut().unwrap().push(c);
-                lines
-            },
-            [escaped_interpolation(c), single_quote_continue(lines)] => {
-                let mut lines = lines;
-                // TODO: don't allocate for every char
-                let c = InterpolatedTextContents::Text(c.to_owned());
-                lines.last_mut().unwrap().push(c);
-                lines
-            },
             [single_quote_char(c), single_quote_continue(lines)] => {
                 let mut lines = lines;
                 if c == "\n" || c == "\r\n" {
@@ -435,38 +428,41 @@ impl Parsers {
         ))
     }
 
+    #[alias(expression)]
     fn builtin<E: Clone>(input: ParseInput<Rule>) -> ParseResult<Expr<E>> {
         let s = input.as_str();
-        let span = input.as_span();
-        Ok(spanned(
-            span,
-            match crate::Builtin::parse(s) {
-                Some(b) => Builtin(b),
-                None => {
-                    match s {
-                        "True" => BoolLit(true),
-                        "False" => BoolLit(false),
-                        "Type" => Const(crate::Const::Type),
-                        "Kind" => Const(crate::Const::Kind),
-                        "Sort" => Const(crate::Const::Sort),
-                        _ => Err(input
-                            .error(format!("Unrecognized builtin: '{}'", s)))?,
-                    }
-                }
+        let e = match crate::Builtin::parse(s) {
+            Some(b) => Builtin(b),
+            None => match s {
+                "True" => BoolLit(true),
+                "False" => BoolLit(false),
+                "Type" => Const(crate::Const::Type),
+                "Kind" => Const(crate::Const::Kind),
+                "Sort" => Const(crate::Const::Sort),
+                _ => Err(input.error(format!("Unrecognized builtin: '{}'", s)))?,
             },
-        ))
+        };
+        Ok(spanned(input.as_span(), e))
     }
 
-    fn NaN(_: ParseInput<Rule>) -> ParseResult<()> {
-        Ok(())
+    #[alias(double_literal)]
+    fn NaN(_input: ParseInput<Rule>) -> ParseResult<core::Double> {
+        Ok(std::f64::NAN.into())
     }
-    fn minus_infinity_literal(_: ParseInput<Rule>) -> ParseResult<()> {
-        Ok(())
+    #[alias(double_literal)]
+    fn minus_infinity_literal(
+        _input: ParseInput<Rule>,
+    ) -> ParseResult<core::Double> {
+        Ok(std::f64::NEG_INFINITY.into())
     }
-    fn plus_infinity_literal(_: ParseInput<Rule>) -> ParseResult<()> {
-        Ok(())
+    #[alias(double_literal)]
+    fn plus_infinity_literal(
+        _input: ParseInput<Rule>,
+    ) -> ParseResult<core::Double> {
+        Ok(std::f64::INFINITY.into())
     }
 
+    #[alias(double_literal)]
     fn numeric_double_literal(
         input: ParseInput<Rule>,
     ) -> ParseResult<core::Double> {
@@ -481,15 +477,6 @@ impl Parsers {
         }
     }
 
-    fn double_literal(input: ParseInput<Rule>) -> ParseResult<core::Double> {
-        Ok(parse_children!(input;
-            [numeric_double_literal(n)] => n,
-            [minus_infinity_literal(_)] => std::f64::NEG_INFINITY.into(),
-            [plus_infinity_literal(_)] => std::f64::INFINITY.into(),
-            [NaN(_)] => std::f64::NAN.into(),
-        ))
-    }
-
     fn natural_literal(input: ParseInput<Rule>) -> ParseResult<core::Natural> {
         input
             .as_str()
@@ -506,13 +493,13 @@ impl Parsers {
             .map_err(|e| input.error(format!("{}", e)))
     }
 
+    #[alias(expression, shortcut = true)]
     fn identifier<E: Clone>(input: ParseInput<Rule>) -> ParseResult<Expr<E>> {
-        let span = input.as_span();
         Ok(parse_children!(input;
             [variable(v)] => {
-                spanned(span, Var(v))
+                spanned(input.as_span(), Var(v))
             },
-            [builtin(e)] => e,
+            [expression(e)] => e,
         ))
     }
 
@@ -527,42 +514,40 @@ impl Parsers {
         ))
     }
 
+    #[alias(path_component)]
     fn unquoted_path_component<'a>(
         input: ParseInput<'a, Rule>,
-    ) -> ParseResult<&'a str> {
-        Ok(input.as_str())
+    ) -> ParseResult<String> {
+        Ok(input.as_str().to_string())
     }
+    #[alias(path_component)]
     fn quoted_path_component<'a>(
         input: ParseInput<'a, Rule>,
-    ) -> ParseResult<&'a str> {
-        Ok(input.as_str())
-    }
-    fn path_component(input: ParseInput<Rule>) -> ParseResult<String> {
-        Ok(parse_children!(input;
-            [unquoted_path_component(s)] => s.to_string(),
-            [quoted_path_component(s)] => {
-                const RESERVED: &percent_encoding::AsciiSet =
-                    &percent_encoding::CONTROLS
-                    .add(b'=').add(b':').add(b'/').add(b'?')
-                    .add(b'#').add(b'[').add(b']').add(b'@')
-                    .add(b'!').add(b'$').add(b'&').add(b'\'')
-                    .add(b'(').add(b')').add(b'*').add(b'+')
-                    .add(b',').add(b';');
-                s.chars()
-                    .map(|c| {
-                        // Percent-encode ascii chars
-                        if c.is_ascii() {
-                            percent_encoding::utf8_percent_encode(
-                                &c.to_string(),
-                                RESERVED,
-                            ).to_string()
-                        } else {
-                            c.to_string()
-                        }
-                    })
-                    .collect()
-            },
-        ))
+    ) -> ParseResult<String> {
+        #[rustfmt::skip]
+        const RESERVED: &percent_encoding::AsciiSet =
+            &percent_encoding::CONTROLS
+            .add(b'=').add(b':').add(b'/').add(b'?')
+            .add(b'#').add(b'[').add(b']').add(b'@')
+            .add(b'!').add(b'$').add(b'&').add(b'\'')
+            .add(b'(').add(b')').add(b'*').add(b'+')
+            .add(b',').add(b';');
+        Ok(input
+            .as_str()
+            .chars()
+            .map(|c| {
+                // Percent-encode ascii chars
+                if c.is_ascii() {
+                    percent_encoding::utf8_percent_encode(
+                        &c.to_string(),
+                        RESERVED,
+                    )
+                    .to_string()
+                } else {
+                    c.to_string()
+                }
+            })
+            .collect())
     }
     fn path(input: ParseInput<Rule>) -> ParseResult<Vec<String>> {
         Ok(parse_children!(input;
@@ -572,17 +557,16 @@ impl Parsers {
         ))
     }
 
-    fn local(
+    #[alias(import_type)]
+    fn local<E: Clone>(
         input: ParseInput<Rule>,
-    ) -> ParseResult<(FilePrefix, Vec<String>)> {
+    ) -> ParseResult<ImportLocation<Expr<E>>> {
         Ok(parse_children!(input;
-            [parent_path(l)] => l,
-            [here_path(l)] => l,
-            [home_path(l)] => l,
-            [absolute_path(l)] => l,
+            [local_path((prefix, p))] => ImportLocation::Local(prefix, p),
         ))
     }
 
+    #[alias(local_path)]
     fn parent_path(
         input: ParseInput<Rule>,
     ) -> ParseResult<(FilePrefix, Vec<String>)> {
@@ -590,6 +574,7 @@ impl Parsers {
             [path(p)] => (FilePrefix::Parent, p)
         ))
     }
+    #[alias(local_path)]
     fn here_path(
         input: ParseInput<Rule>,
     ) -> ParseResult<(FilePrefix, Vec<String>)> {
@@ -597,6 +582,7 @@ impl Parsers {
             [path(p)] => (FilePrefix::Here, p)
         ))
     }
+    #[alias(local_path)]
     fn home_path(
         input: ParseInput<Rule>,
     ) -> ParseResult<(FilePrefix, Vec<String>)> {
@@ -604,6 +590,7 @@ impl Parsers {
             [path(p)] => (FilePrefix::Home, p)
         ))
     }
+    #[alias(local_path)]
     fn absolute_path(
         input: ParseInput<Rule>,
     ) -> ParseResult<(FilePrefix, Vec<String>)> {
@@ -649,25 +636,31 @@ impl Parsers {
         Ok(input.as_str().to_owned())
     }
 
-    fn http<E: Clone>(input: ParseInput<Rule>) -> ParseResult<URL<Expr<E>>> {
-        Ok(parse_children!(input;
-        [http_raw(url)] => url,
-        [http_raw(url), import_expression(e)] =>
-            URL { headers: Some(e), ..url },
-        ))
+    #[alias(import_type)]
+    fn http<E: Clone>(
+        input: ParseInput<Rule>,
+    ) -> ParseResult<ImportLocation<Expr<E>>> {
+        Ok(ImportLocation::Remote(parse_children!(input;
+            [http_raw(url)] => url,
+            [http_raw(url), expression(e)] => URL { headers: Some(e), ..url },
+        )))
     }
 
-    fn env(input: ParseInput<Rule>) -> ParseResult<String> {
+    #[alias(import_type)]
+    fn env<E: Clone>(
+        input: ParseInput<Rule>,
+    ) -> ParseResult<ImportLocation<Expr<E>>> {
         Ok(parse_children!(input;
-            [bash_environment_variable(s)] => s,
-            [posix_environment_variable(s)] => s,
+            [environment_variable(v)] => ImportLocation::Env(v),
         ))
     }
+    #[alias(environment_variable)]
     fn bash_environment_variable(
         input: ParseInput<Rule>,
     ) -> ParseResult<String> {
         Ok(input.as_str().to_owned())
     }
+    #[alias(environment_variable)]
     fn posix_environment_variable(
         input: ParseInput<Rule>,
     ) -> ParseResult<String> {
@@ -694,27 +687,11 @@ impl Parsers {
         })
     }
 
-    fn missing(_: ParseInput<Rule>) -> ParseResult<()> {
-        Ok(())
-    }
-
-    fn import_type<E: Clone>(
-        input: ParseInput<Rule>,
+    #[alias(import_type)]
+    fn missing<E: Clone>(
+        _input: ParseInput<Rule>,
     ) -> ParseResult<ImportLocation<Expr<E>>> {
-        Ok(parse_children!(input;
-            [missing(_)] => {
-                ImportLocation::Missing
-            },
-            [env(e)] => {
-                ImportLocation::Env(e)
-            },
-            [http(url)] => {
-                ImportLocation::Remote(url)
-            },
-            [local((prefix, p))] => {
-                ImportLocation::Local(prefix, p)
-            },
-        ))
+        Ok(ImportLocation::Missing)
     }
 
     fn hash(input: ParseInput<Rule>) -> ParseResult<Hash> {
@@ -730,78 +707,65 @@ impl Parsers {
     fn import_hashed<E: Clone>(
         input: ParseInput<Rule>,
     ) -> ParseResult<crate::Import<Expr<E>>> {
+        use crate::Import;
+        let mode = ImportMode::Code;
         Ok(parse_children!(input;
-        [import_type(location)] =>
-            crate::Import {mode: ImportMode::Code, location, hash: None },
-        [import_type(location), hash(h)] =>
-        crate::Import {mode: ImportMode::Code, location, hash: Some(h) },
+            [import_type(location)] => Import { mode, location, hash: None },
+            [import_type(location), hash(h)] => Import { mode, location, hash: Some(h) },
         ))
     }
 
-    fn Text(_: ParseInput<Rule>) -> ParseResult<()> {
-        Ok(())
+    #[alias(import_mode)]
+    fn Text(_input: ParseInput<Rule>) -> ParseResult<ImportMode> {
+        Ok(ImportMode::RawText)
     }
-    fn Location(_: ParseInput<Rule>) -> ParseResult<()> {
-        Ok(())
+    #[alias(import_mode)]
+    fn Location(_input: ParseInput<Rule>) -> ParseResult<ImportMode> {
+        Ok(ImportMode::Location)
     }
 
+    #[alias(expression)]
     fn import<E: Clone>(input: ParseInput<Rule>) -> ParseResult<Expr<E>> {
-        let span = input.as_span();
-        Ok(parse_children!(input;
+        use crate::Import;
+        let import = parse_children!(input;
             [import_hashed(imp)] => {
-                spanned(span, Import(crate::Import {
-                    mode: ImportMode::Code,
-                    ..imp
-                }))
+                Import { mode: ImportMode::Code, ..imp }
             },
-            [import_hashed(imp), Text(_)] => {
-                spanned(span, Import(crate::Import {
-                    mode: ImportMode::RawText,
-                    ..imp
-                }))
+            [import_hashed(imp), import_mode(mode)] => {
+                Import { mode, ..imp }
             },
-            [import_hashed(imp), Location(_)] => {
-                spanned(span, Import(crate::Import {
-                    mode: ImportMode::Location,
-                    ..imp
-                }))
-            },
-        ))
+        );
+        Ok(spanned(input.as_span(), Import(import)))
     }
 
-    fn lambda(_: ParseInput<Rule>) -> ParseResult<()> {
+    fn lambda(_input: ParseInput<Rule>) -> ParseResult<()> {
         Ok(())
     }
-    fn forall(_: ParseInput<Rule>) -> ParseResult<()> {
+    fn forall(_input: ParseInput<Rule>) -> ParseResult<()> {
         Ok(())
     }
-    fn arrow(_: ParseInput<Rule>) -> ParseResult<()> {
+    fn arrow(_input: ParseInput<Rule>) -> ParseResult<()> {
         Ok(())
     }
-    fn merge(_: ParseInput<Rule>) -> ParseResult<()> {
+    fn merge(_input: ParseInput<Rule>) -> ParseResult<()> {
         Ok(())
     }
-    fn assert(_: ParseInput<Rule>) -> ParseResult<()> {
+    fn assert(_input: ParseInput<Rule>) -> ParseResult<()> {
         Ok(())
     }
-    fn if_(_: ParseInput<Rule>) -> ParseResult<()> {
+    fn if_(_input: ParseInput<Rule>) -> ParseResult<()> {
         Ok(())
     }
-    fn in_(_: ParseInput<Rule>) -> ParseResult<()> {
-        Ok(())
-    }
-    fn toMap(_: ParseInput<Rule>) -> ParseResult<()> {
+    fn toMap(_input: ParseInput<Rule>) -> ParseResult<()> {
         Ok(())
     }
 
+    #[alias(expression)]
     fn empty_list_literal<E: Clone>(
         input: ParseInput<Rule>,
     ) -> ParseResult<Expr<E>> {
-        let span = input.as_span();
         Ok(parse_children!(input;
-            [application_expression(e)] => {
-                spanned(span, EmptyListLit(e))
-            },
+            [expression(e)] => spanned(input.as_span(), EmptyListLit(e)),
         ))
     }
 
@@ -816,7 +780,7 @@ impl Parsers {
                     expression(right)] => {
                 spanned(span, BoolIf(cond, left, right))
             },
-            [let_binding(bindings).., in_(()), expression(final_expr)] => {
+            [let_binding(bindings).., expression(final_expr)] => {
                 bindings.rev().fold(
                     final_expr,
                     |acc, x| unspanned(Let(x.0, x.1, x.2, acc))
@@ -826,24 +790,22 @@ impl Parsers {
                     arrow(()), expression(body)] => {
                 spanned(span, Pi(l, typ, body))
             },
-            [operator_expression(typ), arrow(()), expression(body)] => {
+            [expression(typ), arrow(()), expression(body)] => {
                 spanned(span, Pi("_".into(), typ, body))
             },
-            [merge(()), import_expression(x), import_expression(y),
-                    application_expression(z)] => {
+            [merge(()), expression(x), expression(y), expression(z)] => {
                 spanned(span, Merge(x, y, Some(z)))
             },
-            [empty_list_literal(e)] => e,
             [assert(()), expression(x)] => {
                 spanned(span, Assert(x))
             },
-            [toMap(()), import_expression(x), application_expression(y)] => {
+            [toMap(()), expression(x), expression(y)] => {
                 spanned(span, ToMap(x, Some(y)))
             },
-            [operator_expression(e)] => e,
-            [operator_expression(e), expression(annot)] => {
+            [expression(e), expression(annot)] => {
                 spanned(span, Annot(e, annot))
             },
+            [expression(e)] => e,
         ))
     }
 
@@ -858,14 +820,8 @@ impl Parsers {
         ))
     }
 
-    fn List(_: ParseInput<Rule>) -> ParseResult<()> {
-        Ok(())
-    }
-    fn Optional(_: ParseInput<Rule>) -> ParseResult<()> {
-        Ok(())
-    }
-
-    #[prec_climb(application_expression, PRECCLIMBER)]
+    #[alias(expression, shortcut = true)]
+    #[prec_climb(expression, PRECCLIMBER)]
     fn operator_expression<E: Clone>(
         input: ParseInput<Rule>,
         l: Expr<E>,
@@ -894,55 +850,48 @@ impl Parsers {
         Ok(unspanned(BinOp(op, l, r)))
     }
 
-    fn Some_(_: ParseInput<Rule>) -> ParseResult<()> {
+    fn Some_(_input: ParseInput<Rule>) -> ParseResult<()> {
         Ok(())
     }
 
+    #[alias(expression, shortcut = true)]
     fn application_expression<E: Clone>(
         input: ParseInput<Rule>,
     ) -> ParseResult<Expr<E>> {
         Ok(parse_children!(input;
-            [first_application_expression(e)] => e,
-            [first_application_expression(first),
-                    import_expression(rest)..] => {
+            [expression(e)] => e,
+            [expression(first), expression(rest)..] => {
                 rest.fold(first, |acc, e| unspanned(App(acc, e)))
             },
         ))
     }
 
+    #[alias(expression, shortcut = true)]
     fn first_application_expression<E: Clone>(
         input: ParseInput<Rule>,
     ) -> ParseResult<Expr<E>> {
         let span = input.as_span();
         Ok(parse_children!(input;
-            [Some_(()), import_expression(e)] => {
+            [Some_(()), expression(e)] => {
                 spanned(span, SomeLit(e))
             },
-            [merge(()), import_expression(x), import_expression(y)] => {
+            [merge(()), expression(x), expression(y)] => {
                 spanned(span, Merge(x, y, None))
             },
-            [toMap(()), import_expression(x)] => {
+            [toMap(()), expression(x)] => {
                 spanned(span, ToMap(x, None))
             },
-            [import_expression(e)] => e,
-        ))
-    }
-
-    fn import_expression<E: Clone>(
-        input: ParseInput<Rule>,
-    ) -> ParseResult<Expr<E>> {
-        Ok(parse_children!(input;
-            [selector_expression(e)] => e,
-            [import(e)] => e,
+            [expression(e)] => e,
         ))
     }
 
+    #[alias(expression, shortcut = true)]
     fn selector_expression<E: Clone>(
         input: ParseInput<Rule>,
     ) -> ParseResult<Expr<E>> {
         Ok(parse_children!(input;
-            [primitive_expression(e)] => e,
-            [primitive_expression(first), selector(rest)..] => {
+            [expression(e)] => e,
+            [expression(first), selector(rest)..] => {
                 rest.fold(first, |acc, e| unspanned(match e {
                     Either::Left(l) => Field(acc, l),
                     Either::Right(ls) => Projection(acc, ls),
@@ -967,6 +916,7 @@ impl Parsers {
         ))
     }
 
+    #[alias(expression, shortcut = true)]
     fn primitive_expression<E: Clone>(
         input: ParseInput<Rule>,
     ) -> ParseResult<Expr<E>> {
@@ -977,46 +927,41 @@ impl Parsers {
             [integer_literal(n)] => spanned(span, IntegerLit(n)),
             [double_quote_literal(s)] => spanned(span, TextLit(s)),
             [single_quote_literal(s)] => spanned(span, TextLit(s)),
-            [empty_record_type(e)] => e,
-            [empty_record_literal(e)] => e,
-            [non_empty_record_type_or_literal(e)] => e,
-            [union_type(e)] => e,
-            [non_empty_list_literal(e)] => e,
-            [identifier(e)] => e,
             [expression(e)] => e,
         ))
     }
 
+    #[alias(expression)]
     fn empty_record_literal<E: Clone>(
         input: ParseInput<Rule>,
     ) -> ParseResult<Expr<E>> {
-        let span = input.as_span();
-        Ok(spanned(span, RecordLit(Default::default())))
+        Ok(spanned(input.as_span(), RecordLit(Default::default())))
     }
 
+    #[alias(expression)]
     fn empty_record_type<E: Clone>(
         input: ParseInput<Rule>,
     ) -> ParseResult<Expr<E>> {
-        let span = input.as_span();
-        Ok(spanned(span, RecordType(Default::default())))
+        Ok(spanned(input.as_span(), RecordType(Default::default())))
     }
 
+    #[alias(expression)]
     fn non_empty_record_type_or_literal<E: Clone>(
         input: ParseInput<Rule>,
     ) -> ParseResult<Expr<E>> {
-        let span = input.as_span();
-        Ok(parse_children!(input;
+        let e = parse_children!(input;
             [label(first_label), non_empty_record_type(rest)] => {
                 let (first_expr, mut map) = rest;
                 map.insert(first_label, first_expr);
-                spanned(span, RecordType(map))
+                RecordType(map)
             },
             [label(first_label), non_empty_record_literal(rest)] => {
                 let (first_expr, mut map) = rest;
                 map.insert(first_label, first_expr);
-                spanned(span, RecordLit(map))
+                RecordLit(map)
             },
-        ))
+        );
+        Ok(spanned(input.as_span(), e))
     }
 
     fn non_empty_record_type<E: Clone>(
@@ -1055,19 +1000,16 @@ impl Parsers {
         ))
     }
 
+    #[alias(expression)]
     fn union_type<E: Clone>(input: ParseInput<Rule>) -> ParseResult<Expr<E>> {
-        let span = input.as_span();
-        Ok(parse_children!(input;
-            [empty_union_type(_)] => {
-                spanned(span, UnionType(Default::default()))
-            },
-            [union_type_entry(entries)..] => {
-                spanned(span, UnionType(entries.collect()))
-            },
-        ))
+        let map = parse_children!(input;
+            [empty_union_type(_)] => Default::default(),
+            [union_type_entry(entries)..] => entries.collect(),
+        );
+        Ok(spanned(input.as_span(), UnionType(map)))
     }
 
-    fn empty_union_type(_: ParseInput<Rule>) -> ParseResult<()> {
+    fn empty_union_type(_input: ParseInput<Rule>) -> ParseResult<()> {
         Ok(())
     }
 
@@ -1080,13 +1022,13 @@ impl Parsers {
         ))
     }
 
+    #[alias(expression)]
     fn non_empty_list_literal<E: Clone>(
         input: ParseInput<Rule>,
     ) -> ParseResult<Expr<E>> {
-        let span = input.as_span();
         Ok(parse_children!(input;
             [expression(items)..] => spanned(
-                span,
+                input.as_span(),
                 NEListLit(items.collect())
             )
         ))