schala/schala-lang/language/src/parsing.rs

//! # Parsing
//! This module is where the recursive-descent parsing methods live.
//!
//!
//! # Schala EBNF Grammar
//! This document is the authoritative grammar of Schala, represented in something approximating
//! Extended Backus-Naur form. Terminal productions are in "double quotes", or UPPERCASE
//! if they represent a class of tokens rather than an specific string, or are otherwise
//! unreprsentable in ASCII.
//!
//! ## Top level structure
//!
//! ```text
//! program := (statement delimiter)* EOF
//! delimiter := NEWLINE | ";"
//! statement := expression | declaration | import
//! block := "{" (statement delimiter)* "}"
//! declaration := type_declaration | func_declaration | binding_declaration | impl_declaration
//! ```
//! ## Declarations
//!
//! ### Types
//! ```text
//! type_declaration := "type" type_declaration_body
//! type_declaration_body := "alias" type_alias | "mut"? type_singleton_name "=" type_body
//! type_alias := IDENTIFIER "=" type_name
//! type_body := variant_specifier ("|" variant_specifier)*
//! variant_specifier := IDENTIFIER | IDENTIFIER "{" typed_identifier_list "}" | IDENTIFIER "(" type_name* ")"
//! typed_identifier_list := typed_identifier*
//! typed_identifier := IDENTIFIER type_anno
//! ```
//! ### Functions
//!
//! ```text
//! func_declaration := func_signature func_body
//! func_body := ε | nonempty_func_body
//! nonempty_func_body := "{" (statement delimiter)* "}"
//! func_signature := "fn" func_name formal_param_list type_anno+
//! func_name := IDENTIFIER | operator
//! formal_param_list := "(" (formal_param ",")* ")"
//! formal_param := IDENTIFIER type_anno+
//! ```
//!
//! ### Variable bindings
//! ```text binding_declaration := "let" "mut"? IDENTIFIER "=" expresion```
//!
//! ### Interfaces
//!
//! ```text
//! interface_declaration := "interface" type_singleton_name signature_block
//! impl_declaration := "impl" type_singleton_name decl_block | "impl" type_singleton_name "for" type_name decl_block
//! decl_block := "{" (func_declaration)* "}"
//! signature_block := "{" (func_signature)* "}"
//! ```
//!
//! ### Type Annotations
//!
//! ```text
//! type_anno := ":" type_name
//! type_name := type_singleton_name | "(" type_names ")"
//! type_names := ε | type_name (, type_name)*
//! type_singleton_name = IDENTIFIER (type_params)*
//! type_params := "<" type_name (, type_name)* ">"
//! ```
//!
//! ## Expressions
//! ```text
//! expression := precedence_expr type_anno+
//! precedence_expr := prefix_expr
//! prefix_expr := prefix_op call_expr
//! prefix_op := "+" | "-" | "!" | "~"
//! call_expr := index_expr ( "(" invocation_list ")" )* | ε
//! invocation_list := invocation_argument ("," invocation_argument)* | ε
//! invocation_argument := expression | IDENTIFIER "=" expression | "_"
//! index_expr := primary ( "[" (expression ("," (expression)* | ε) "]" )*
//! primary := literal | paren_expr | if_expr | for_expr | while_expr | identifier_expr | lambda_expr | anonymous_struct | list_expr
//! expr_or_block := "{" (statement delimiter)* "}" | expr
//! ```
//!
//! ### Primary expressions
//!
//! ```text
//! list_expr := "[" (expression, ",")* "]"
//! lambda_expr := "\\" lambda_param_list type_anno+ nonempty_func_body
//! lambda_param_list := formal_param_list |  formal_param
//! paren_expr := "(" paren_inner ")"
//! paren_inner := (expression ",")*
//! identifier_expr := qualified_identifier | named_struct
//! qualified_identifier := IDENTIFIER ("::" IDENTIFIER)*
//! ```
//!
//! ## Literals
//! ```text
//! literal := "true" | "false" | number_literal | STR_LITERAL
//! named_struct := qualified_identifier record_block
//! record_block :=  "{" (record_entry, ",")* | "}" //TODO support anonymus structs, update syntax
//! record_entry := IDENTIFIER ":" expression
//! anonymous_struct := TODO
//! number_literal := int_literal | float_literal
//! int_literal = ("0x" | "0b") digits
//! float_literal := digits ("." digits)
//! digits := (digit_group underscore*)+
//! digit_group := DIGIT+
//! ```
//!
//! ### Patterns
//! ```text
//! pattern := "(" (pattern, ",")* ")" | simple_pattern
//! simple_pattern := pattern_literal | record_pattern | tuple_struct_pattern
//! pattern_literal := "true" | "false" | signed_number_literal | STR_LITERAL | qualified_identifier
//! signed_number_literal := "-"? number_literal
//! record_pattern := qualified_identifier "{" (record_pattern_entry, ",")* "}"
//! record_pattern_entry := IDENTIFIER | IDENTIFIER ":" Pattern
//! tuple_struct_pattern := qualified_identifier "(" (pattern, ",")* ")"
//! ```
//!
//! ### If-expressions
//! ```text
//! if_expr := "if" discriminator ("then" condititional | "is" simple_pattern_match | guard_block)
//! discriminator := precedence_expr (operator)+
//! conditional := expr_or_block else_clause
//! simple_pattern_match := pattern "then" conditional
//! else_clause := ε | "else" expr_or_block
//! guard_block := "{" (guard_arm, ",")* "}"
//! guard_arm := guard "then" expr_or_block
//! guard := "is" pattern | (operator)+ precedence_expr
//! ```
//!
//! ### While expressions
//! ```text
//! while_expr := "while" while_cond "{" (statement delimiter)* "}"
//! while_cond := ε | expression | expression "is"  pattern //TODO maybe is-expresions should be primary
//! ```
//!
//! //TODO this implies there must be at least one enumerator, which the parser doesn"t support right
//! //this second, and maybe should fail later anyway
//! ### For-expressions
//! ```text
//! for_expr := "for" (enumerator | "{" enumerators "}") for_expr_body
//! for_expr_body := "return" expression |  "{" (statement delimiter)* "}"
//! enumerators := enumerator ("," enumerators)*
//! enumerator :=  identifier "<-" expression | identifier "=" expression //TODO add guards, etc.
//! ```
//! ## Imports
//! ```text
//! import := 'import' IDENTIFIER (:: IDENTIFIER)*  import_suffix
//! import_suffix := ε | '::{' IDENTIFIER (, IDENTIFIER)* '}' | '*' //TODO add qualified, exclusions, etc.
//! ```
mod test;

use std::rc::Rc;
use std::str::FromStr;

use crate::tokenizing::*;
use crate::tokenizing::Kw::*;
use crate::tokenizing::TokenKind::*;

use crate::ast::*;

/// Represents a parsing error
#[derive(Debug)]
pub struct ParseError {
  pub production_name: Option<String>,
  pub msg: String,
  pub token: Token
}

impl ParseError {
  fn new_with_token<T, M>(msg: M, token: Token) -> ParseResult<T> where M: Into<String> {
    Err(ParseError { msg: msg.into(), token, production_name: None })
  }
}

/// Represents either a successful parsing result or a ParseError
pub type ParseResult<T> = Result<T, ParseError>;

#[derive(Debug)]
pub struct ParseRecord {
  production_name: String,
  next_token: String,
  level: u32,
}

/// Main data structure for doing parsing.
pub struct Parser {
  token_handler: TokenHandler,
  parse_record: Vec<ParseRecord>,
  parse_level: u32,
  restrictions: ParserRestrictions,
  id_store: ItemIdStore,
}

struct ParserRestrictions {
  no_struct_literal: bool
}

struct TokenHandler {
  tokens: Vec<Token>,
  idx: usize,
  end_of_file: (usize, usize),
}

impl TokenHandler {
  fn new(tokens: Vec<Token>) -> TokenHandler {
    let end_of_file = match tokens.last() {
      None => (0, 0),
      Some(t) => (t.line_num, t.char_num)
    };
    TokenHandler { idx: 0, tokens, end_of_file }
  }

  fn peek_kind(&mut self) -> TokenKind {
    self.peek().kind
  }

  fn peek_kind_n(&mut self, n: usize) -> TokenKind {
    self.peek_n(n).kind
  }
  fn peek(&mut self) -> Token {
    self.tokens.get(self.idx).map(|t: &Token| { t.clone()}).unwrap_or(Token { kind: TokenKind::EOF, line_num: self.end_of_file.0, char_num: self.end_of_file.1})
  }
  /// calling peek_n(0) is the same thing as peek()
  fn peek_n(&mut self, n: usize) -> Token {
    self.tokens.get(self.idx + n).map(|t: &Token| { t.clone()}).unwrap_or(Token { kind: TokenKind::EOF, line_num: self.end_of_file.0, char_num: self.end_of_file.1})
  }
  fn next(&mut self) -> Token {
    self.idx += 1;
    self.tokens.get(self.idx - 1).map(|t: &Token| { t.clone() }).unwrap_or(Token { kind: TokenKind::EOF, line_num: self.end_of_file.0, char_num: self.end_of_file.1})
  }
}

impl Parser {
  /// Create a new parser initialized with some tokens.
  pub fn new(initial_input: Vec<Token>) -> Parser {
    Parser {
      token_handler: TokenHandler::new(initial_input),
      parse_record: vec![],
      parse_level: 0,
      restrictions: ParserRestrictions { no_struct_literal: false },
      id_store: ItemIdStore::new(),
    }
  }

  /// Parse all loaded tokens up to this point.
  pub fn parse(&mut self) -> ParseResult<AST> {
    self.program()
  }

  /*
  pub fn parse_with_new_tokens(&mut self, new_tokens: Vec<Token>) -> ParseResult<AST> {

  }
  */

  pub fn format_parse_trace(&self) -> String {
    let mut buf = String::new();
    for r in self.parse_record.iter() {
      let mut indent = String::new();
      for _ in 0..r.level {
        indent.push(' ');
      }
      buf.push_str(&format!("{}Production `{}`, token: {}\n", indent, r.production_name, r.next_token))
    }
    buf
  }
}

macro_rules! print_token_pattern {
  ($tokenpattern:pat) => { stringify!($tokenpattern) }
}

macro_rules! expect {
  ($self:expr, $token_kind:pat) => { expect!($self, $token_kind if true) };
  ($self:expr, $expected_kind:pat if $cond:expr) => {
    {
      let tok = $self.token_handler.peek();
      match tok.get_kind() {
        $expected_kind if $cond => $self.token_handler.next(),
        actual_kind => {
          let msg = format!("Expected {}, got {:?}", print_token_pattern!($expected_kind), actual_kind);
          return ParseError::new_with_token(msg, tok);
        }
      }
    }
  }
}

macro_rules! delimited {
  ($self:expr, $start:pat, $parse_fn:ident, $( $delim:pat )|+, $end:pat, nonstrict) => {
    delimited!($self, $start, $parse_fn, $( $delim )|*, $end, false)
  };
  ($self:expr, $start:pat, $parse_fn:ident, $( $delim:pat )|+, $end:pat) => {
    delimited!($self, $start, $parse_fn, $( $delim )|*, $end, true)
  };
  ($self:expr, $start:pat, $parse_fn:ident, $( $delim:pat )|+, $end:pat, $strictness:expr) => {
    {
      expect!($self, $start);
      let mut acc = vec![];
      loop {
        let peek = $self.token_handler.peek();
        match peek.get_kind() {
          $end | EOF => break,
          _ => (),
        }
        if !$strictness {
          match peek.get_kind() {
            $( $delim )|* => { $self.token_handler.next(); continue },
            _ => ()
          }
        }
        acc.push($self.$parse_fn()?);
        match $self.token_handler.peek().get_kind() {
          $( $delim )|* => { $self.token_handler.next(); continue },
          _ if $strictness => break,
          _ => continue,
        };
      }
      expect!($self, $end);
      acc
    }
  };
}


impl Parser {
  /// `program := (statement delimiter)* EOF`
  /// `delimiter := NEWLINE | ';'`
  #[recursive_descent_method]
  fn program(&mut self) -> ParseResult<AST> {
    let mut statements = Vec::new();
    loop {
      match self.token_handler.peek().get_kind() {
        EOF => break,
        Newline | Semicolon => {
          self.token_handler.next();
          continue;
        },
        _ => statements.push(
          self.statement()?
        ),
      }
    }
    Ok(AST { id: self.id_store.fresh(), statements })
  }

  /// `statement := expression | declaration`
  #[recursive_descent_method]
  fn statement(&mut self) -> ParseResult<Statement> {
    //TODO handle error recovery here
    let kind = match self.token_handler.peek().get_kind() {
      Keyword(Type) => self.type_declaration().map(|decl| { StatementKind::Declaration(decl) }),
      Keyword(Func)=> self.func_declaration().map(|func| { StatementKind::Declaration(func) }),
      Keyword(Let) => self.binding_declaration().map(|decl| StatementKind::Declaration(decl)),
      Keyword(Interface) => self.interface_declaration().map(|decl| StatementKind::Declaration(decl)),
      Keyword(Impl) => self.impl_declaration().map(|decl| StatementKind::Declaration(decl)),
      Keyword(Import) => self.import_declaration().map(|spec| StatementKind::Import(spec)),
      _ => self.expression().map(|expr| { StatementKind::Expression(expr) } ),
    }?;
    Ok(Statement { kind, id: self.id_store.fresh() })
  }

  #[recursive_descent_method]
  fn type_declaration(&mut self) -> ParseResult<Declaration> {
    expect!(self, Keyword(Type));
    self.type_declaration_body()
  }

  #[recursive_descent_method]
  fn type_declaration_body(&mut self) -> ParseResult<Declaration> {
    if let Keyword(Alias) = self.token_handler.peek_kind() {
      self.type_alias()
    } else {
      let mutable = if let Keyword(Mut) = self.token_handler.peek_kind() {
        self.token_handler.next();
        true
      } else {
        false
      };
      let name = self.type_singleton_name()?;
      expect!(self, Equals);
      let body = self.type_body()?;
      Ok(Declaration::TypeDecl { name, body, mutable})
    }
  }

  #[recursive_descent_method]
  fn type_alias(&mut self) -> ParseResult<Declaration> {
    expect!(self, Keyword(Alias));
    let alias = self.identifier()?;
    expect!(self, Equals);
    let original = self.identifier()?;
    Ok(Declaration::TypeAlias { alias, original })
  }

  #[recursive_descent_method]
  fn type_body(&mut self) -> ParseResult<TypeBody> {
    let mut variants = Vec::new();
    variants.push(self.variant_specifier()?);
    loop {
      if let Pipe = self.token_handler.peek_kind() {
        self.token_handler.next();
        variants.push(self.variant_specifier()?);
      } else {
        break;
      }
    }
    Ok(TypeBody(variants))
  }

  #[recursive_descent_method]
  fn variant_specifier(&mut self) -> ParseResult<Variant> {
    use self::Variant::*;

    let name = self.identifier()?;
    match self.token_handler.peek_kind() {
      LParen => {
        let tuple_members = delimited!(self, LParen, type_name, Comma, RParen);
        Ok(TupleStruct(name, tuple_members))
      },
      LCurlyBrace => {
        let typed_identifier_list = delimited!(self, LCurlyBrace, typed_identifier, Comma, RCurlyBrace);
        Ok(Record {name, members: typed_identifier_list })
      },
      _ => Ok(UnitStruct(name))
    }
  }

  #[recursive_descent_method]
  fn typed_identifier(&mut self) -> ParseResult<(Rc<String>, TypeIdentifier)> {
    let identifier = self.identifier()?;
    expect!(self, Colon);
    let type_name = self.type_name()?;
    Ok((identifier, type_name))
  }

  #[recursive_descent_method]
  fn func_declaration(&mut self) -> ParseResult<Declaration> {
    let signature = self.func_signature()?;
    if let LCurlyBrace = self.token_handler.peek_kind() {
      let statements = self.nonempty_func_body()?;
      Ok(Declaration::FuncDecl(signature, statements))
    } else {
      Ok(Declaration::FuncSig(signature))
    }
  }

  #[recursive_descent_method]
  fn func_signature(&mut self) -> ParseResult<Signature> {
    expect!(self, Keyword(Func));
    let (name, operator) = match self.token_handler.peek_kind() {
      Operator(s) => {
        let name = s.clone();
        self.token_handler.next();
        (name, true)
      },
      _ => (self.identifier()?, false)
    };
    let params = self.formal_param_list()?;
    let type_anno = match self.token_handler.peek_kind() {
      Colon => Some(self.type_anno()?),
      _ => None,
    };
    Ok(Signature { name, operator, params, type_anno })
  }

  #[recursive_descent_method]
  fn nonempty_func_body(&mut self) -> ParseResult<Vec<Statement>> {
    let statements = delimited!(self, LCurlyBrace, statement, Newline | Semicolon, RCurlyBrace, nonstrict);
    Ok(statements)
  }

  #[recursive_descent_method]
  fn formal_param_list(&mut self) -> ParseResult<Vec<FormalParam>> {
    Ok(delimited!(self, LParen, formal_param, Comma, RParen))
  }

  //TODO needs to support default values
  #[recursive_descent_method]
  fn formal_param(&mut self) -> ParseResult<FormalParam> {
    let name = self.identifier()?;
    let anno = match self.token_handler.peek_kind() {
      Colon => Some(self.type_anno()?),
      _ => None
    };
    let default = match self.token_handler.peek_kind() {
      Equals => {
        self.token_handler.next();
        Some(self.expression()?)
      },
      _ => None
    };
    Ok(FormalParam { name, anno, default })
  }

  #[recursive_descent_method]
  fn binding_declaration(&mut self) -> ParseResult<Declaration> {
    expect!(self, Keyword(Kw::Let));
    let constant = match self.token_handler.peek_kind() {
      Keyword(Kw::Mut) => {
        self.token_handler.next();
        false 
      }
      _ => true
    };
    let name = self.identifier()?;
    let type_anno = if let Colon = self.token_handler.peek_kind() {
      Some(self.type_anno()?)
    } else {
      None
    };

    expect!(self, Equals);
    let expr = self.expression()?;

    Ok(Declaration::Binding { name, constant, type_anno, expr })
  }

  #[recursive_descent_method]
  fn interface_declaration(&mut self) -> ParseResult<Declaration> {
    expect!(self, Keyword(Interface));
    let name = self.identifier()?;
    let signatures = self.signature_block()?;
    Ok(Declaration::Interface { name, signatures })
  }

  #[recursive_descent_method]
  fn signature_block(&mut self) -> ParseResult<Vec<Signature>> {
    Ok(delimited!(self, LCurlyBrace, func_signature, Newline | Semicolon, RCurlyBrace, nonstrict))
  }

  #[recursive_descent_method]
  fn impl_declaration(&mut self) -> ParseResult<Declaration> {
    expect!(self, Keyword(Impl));
    let first = self.type_singleton_name()?;
    let second = if let Keyword(For) = self.token_handler.peek_kind() {
      self.token_handler.next();
      Some(self.type_name()?)
    } else {
      None
    };

    let block = self.decl_block()?;
    Ok(match (first, second) {
      (interface_name, Some(type_name)) =>
        Declaration::Impl { type_name, interface_name: Some(interface_name), block },
      (type_singleton_name, None) =>
        Declaration::Impl { type_name: TypeIdentifier::Singleton(type_singleton_name), interface_name: None, block }
    })
  }

  #[recursive_descent_method]
  fn decl_block(&mut self) -> ParseResult<Vec<Declaration>> {
    Ok(delimited!(self, LCurlyBrace, func_declaration, Newline | Semicolon, RCurlyBrace, nonstrict))
  }

  #[recursive_descent_method]
  fn expression(&mut self) -> ParseResult<Expression> {
    let mut expr_body = self.precedence_expr(BinOp::min_precedence())?;
    let type_anno = match self.token_handler.peek_kind() {
      Colon => Some(self.type_anno()?),
      _ => None
    };
    if let Some(_) = expr_body.type_anno {
      return ParseError::new_with_token("Bad parse state encountered", self.token_handler.peek());
    }
    expr_body.type_anno = type_anno;
    Ok(expr_body)
  }

  #[recursive_descent_method]
  fn type_anno(&mut self) -> ParseResult<TypeIdentifier> {
    expect!(self, Colon);
    self.type_name()
  }

  #[recursive_descent_method]
  fn type_name(&mut self) -> ParseResult<TypeIdentifier> {
    use self::TypeIdentifier::*;
    Ok(match self.token_handler.peek_kind() {
      LParen => Tuple(delimited!(self, LParen, type_name, Comma, RParen)),
      _ => Singleton(self.type_singleton_name()?),
    })
  }

  #[recursive_descent_method]
  fn type_singleton_name(&mut self) -> ParseResult<TypeSingletonName> {
    Ok(TypeSingletonName {
      name: self.identifier()?,
      params: match self.token_handler.peek_kind() {
        LAngleBracket => delimited!(self, LAngleBracket, type_name, Comma, RAngleBracket),
        _ => vec![],
      }
    })
  }

  // this implements Pratt parsing, see http://journal.stuffwithstuff.com/2011/03/19/pratt-parsers-expression-parsing-made-easy/
  fn precedence_expr(&mut self, precedence: i32) -> ParseResult<Expression> {
    let record = ParseRecord {
      production_name: "precedence_expr".to_string(),
      next_token: format!("{}", self.token_handler.peek().to_string_with_metadata()),
      level: self.parse_level,
    };
    self.parse_level += 1;
    self.parse_record.push(record);

    let mut lhs = self.prefix_expr()?;
    loop {
      let new_precedence = match BinOp::get_precedence_from_token(&self.token_handler.peek_kind()) {
        Some(p) => p,
        None => break,
      };

      if precedence >= new_precedence {
        break;
      }
      let next_tok = self.token_handler.next();
      let operation = match BinOp::from_sigil_token(&next_tok.kind) {
        Some(sigil) => sigil,
        None => unreachable!()
      };
      let rhs = self.precedence_expr(new_precedence)?;
      lhs = Expression::new(self.id_store.fresh(), ExpressionKind::BinExp(operation, bx!(lhs), bx!(rhs)));
    }
    self.parse_level -= 1;
    Ok(lhs)
  }

  #[recursive_descent_method]
  fn prefix_expr(&mut self) -> ParseResult<Expression> {
    match self.token_handler.peek_kind() {
      Operator(ref op) if PrefixOp::is_prefix(&*op) => {
        let sigil = match self.token_handler.next().kind {
          Operator(op) => op,
          _ => unreachable!(),
        };
        let expr = self.primary()?;
        let prefix_op = PrefixOp::from_str(sigil.as_str()).unwrap();
        Ok(Expression::new(
          self.id_store.fresh(),
          ExpressionKind::PrefixExp(prefix_op, bx!(expr))
        ))
      },
      _ => self.call_expr()
    }
  }

  #[recursive_descent_method]
  fn call_expr(&mut self) -> ParseResult<Expression> {
    let mut expr = self.index_expr()?;
    while let LParen = self.token_handler.peek_kind() {
      let arguments = delimited!(self, LParen, invocation_argument, Comma, RParen);
      expr = Expression::new(self.id_store.fresh(), ExpressionKind::Call { f: bx!(expr), arguments }); //TODO no type anno is incorrect
    }

    Ok(expr)
  }

  #[recursive_descent_method]
  fn invocation_argument(&mut self) -> ParseResult<InvocationArgument> {
    Ok(match self.token_handler.peek_kind() {
      Underscore => {
        self.token_handler.next();
        InvocationArgument::Ignored
      },
      Identifier(s) => {
        match self.token_handler.peek_kind_n(1) {
          Equals => {
            self.token_handler.next();
            self.token_handler.next();
            let expr = self.expression()?;
            InvocationArgument::Keyword { name: s.clone(), expr }
          },
          _ => {
            let expr = self.expression()?;
            InvocationArgument::Positional(expr)
          }
        }
      },
      _ => InvocationArgument::Positional(self.expression()?)
    })
  }

  #[recursive_descent_method]
  fn index_expr(&mut self) -> ParseResult<Expression> {
    let primary = self.primary()?;
    Ok(if let LSquareBracket = self.token_handler.peek_kind() {
      let indexers = delimited!(self, LSquareBracket, expression, Comma, RSquareBracket);
      Expression::new(self.id_store.fresh(), ExpressionKind::Index {
        indexee: bx!(Expression::new(self.id_store.fresh(), primary.kind)),
        indexers,
      })
    } else {
      primary
    })
  }

  #[recursive_descent_method]
  fn primary(&mut self) -> ParseResult<Expression> {
    match self.token_handler.peek_kind() {
      LCurlyBrace => self.curly_brace_expr(),
      Backslash => self.lambda_expr(),
      LParen => self.paren_expr(),
      LSquareBracket => self.list_expr(),
      Keyword(Kw::If) => self.if_expr(),
      Keyword(Kw::For) => self.for_expr(),
      Keyword(Kw::While) => self.while_expr(),
      Identifier(_) => self.identifier_expr(),
      _ => self.literal(),
    }
  }

  #[recursive_descent_method]
  fn list_expr(&mut self) -> ParseResult<Expression> {
    let exprs = delimited!(self, LSquareBracket, expression, Comma, RSquareBracket);
    Ok(Expression::new(self.id_store.fresh(), ExpressionKind::ListLiteral(exprs)))
  }

  #[recursive_descent_method]
  fn curly_brace_expr(&mut self) -> ParseResult<Expression> {
    ParseError::new_with_token("Not implemented", self.token_handler.peek())
  }

  #[recursive_descent_method]
  fn lambda_expr(&mut self) -> ParseResult<Expression> {
    expect!(self, Backslash);
    let params = self.lambda_param_list()?;
    let type_anno = match self.token_handler.peek_kind() {
      Colon => Some(self.type_anno()?),
      _ => None,
    };
    let body = self.nonempty_func_body()?;
    Ok(Expression::new(self.id_store.fresh(), ExpressionKind::Lambda { params, type_anno, body })) //TODO need to handle types somehow
  }

  #[recursive_descent_method]
  fn lambda_param_list(&mut self) -> ParseResult<Vec<FormalParam>> {
    if let LParen = self.token_handler.peek_kind() {
      self.formal_param_list()
    } else {
      let single_param = self.formal_param()?;
      Ok(vec![single_param])
    }
  }

  #[recursive_descent_method]
  fn paren_expr(&mut self) -> ParseResult<Expression> {
    use self::ExpressionKind::*;
    let old_struct_value = self.restrictions.no_struct_literal;
    self.restrictions.no_struct_literal = false;
    let output = {
      let mut inner = delimited!(self, LParen, expression, Comma, RParen);
      match inner.len() {
        0 => Ok(Expression::new(self.id_store.fresh(), TupleLiteral(vec![]))),
        1 => Ok(inner.pop().unwrap()),
        _ => Ok(Expression::new(self.id_store.fresh(), TupleLiteral(inner)))
      }
    };
    self.restrictions.no_struct_literal = old_struct_value;
    output
  }

  #[recursive_descent_method]
  fn identifier_expr(&mut self) -> ParseResult<Expression> {
    use self::ExpressionKind::*;
    let components = self.qualified_identifier()?;
    let qualified_identifier = QualifiedName { id: self.id_store.fresh(), components };
    Ok(match self.token_handler.peek_kind() {
      LCurlyBrace if !self.restrictions.no_struct_literal => {
        let fields = self.record_block()?;
        Expression::new(self.id_store.fresh(), NamedStruct { name: qualified_identifier, fields })
      },
      _ => Expression::new(self.id_store.fresh(), Value(qualified_identifier))
    })
  }

  #[recursive_descent_method]
  fn qualified_identifier(&mut self) -> ParseResult<Vec<Rc<String>>> {
    let mut components = vec![self.identifier()?];
    loop {
      match (self.token_handler.peek_kind(), self.token_handler.peek_kind_n(1)) {
        (Colon, Colon) => {
          self.token_handler.next(); self.token_handler.next();
          components.push(self.identifier()?);
        },
        _ => break,
      }
    }
    Ok(components)
  }

  #[recursive_descent_method]
  fn record_block(&mut self) -> ParseResult<Vec<(Rc<String>, Expression)>> {
    Ok(delimited!(self, LCurlyBrace, record_entry, Comma, RCurlyBrace))
  }

  #[recursive_descent_method]
  fn record_entry(&mut self) -> ParseResult<(Rc<String>, Expression)> {
    let field_name = self.identifier()?;
    expect!(self, Colon);
    let value = self.expression()?;
    Ok((field_name, value))
  }

  #[recursive_descent_method]
  fn if_expr(&mut self) -> ParseResult<Expression> {
    expect!(self, Keyword(Kw::If));
    let discriminator = Box::new({
      self.restrictions.no_struct_literal = true;
      let x = self.discriminator();
      self.restrictions.no_struct_literal = false;
      x?
    });

    let body = Box::new(match self.token_handler.peek_kind() {
      Keyword(Kw::Then) => self.conditional()?,
      Keyword(Kw::Is) => self.simple_pattern_match()? ,
      _ => self.guard_block()?
    });

    Ok(Expression::new(self.id_store.fresh(), ExpressionKind::IfExpression { discriminator, body }))
  }

  #[recursive_descent_method]
  fn discriminator(&mut self) -> ParseResult<Discriminator> {
    let lhs = self.prefix_expr()?;
    let ref next = self.token_handler.peek_kind();
    Ok(if let Some(op) = BinOp::from_sigil_token(next) {
      self.token_handler.next();
      Discriminator::BinOp(lhs, op)
    } else {
      Discriminator::Simple(lhs)
    })
  }

  #[recursive_descent_method]
  fn conditional(&mut self) -> ParseResult<IfExpressionBody> {
    expect!(self, Keyword(Kw::Then));
    let then_clause = self.expr_or_block()?;
    let else_clause = self.else_clause()?;
    Ok(IfExpressionBody::SimpleConditional(then_clause, else_clause))
  }

  #[recursive_descent_method]
  fn simple_pattern_match(&mut self) -> ParseResult<IfExpressionBody> {
    expect!(self, Keyword(Kw::Is));
    let pat = self.pattern()?;
    expect!(self, Keyword(Kw::Then));
    let then_clause = self.expr_or_block()?;
    let else_clause = self.else_clause()?;
    Ok(IfExpressionBody::SimplePatternMatch(pat, then_clause, else_clause))
  }

  #[recursive_descent_method]
  fn else_clause(&mut self) -> ParseResult<Option<Block>> {
    Ok(if let Keyword(Kw::Else) = self.token_handler.peek_kind() {
      self.token_handler.next();
      Some(self.expr_or_block()?)
    } else {
      None
    })
  }

  #[recursive_descent_method]
  fn guard_block(&mut self) -> ParseResult<IfExpressionBody> {
    //TODO - delimited! isn't sophisticated enough to do thisa
    //let guards = delimited!(self, LCurlyBrace, guard_arm, Comma, RCurlyBrace);
    expect!(self, LCurlyBrace);

    let mut guards = vec![];
    loop {
      match self.token_handler.peek_kind() {
        RCurlyBrace | EOF => break,
        Semicolon | Newline => { self.token_handler.next(); continue},
        _ => {
          let guard_arm = self.guard_arm()?;
          guards.push(guard_arm);
          loop {
            match self.token_handler.peek_kind() {
              Semicolon | Newline => { self.token_handler.next(); continue; },
              _ => break,
            }
          }
          if let RCurlyBrace = self.token_handler.peek_kind() {
            break;
          }
          expect!(self, Comma);
        }
      }
    }
    expect!(self, RCurlyBrace);
    Ok(IfExpressionBody::GuardList(guards))
  }

  #[recursive_descent_method]
  fn guard_arm(&mut self) -> ParseResult<GuardArm> {
    let guard = self.guard()?;
    expect!(self, Keyword(Kw::Then));
    let body = self.expr_or_block()?;
    Ok(GuardArm { guard, body })
  }

  #[recursive_descent_method]
  fn guard(&mut self) -> ParseResult<Guard> {
    Ok(match self.token_handler.peek_kind() {
      Keyword(Kw::Is) => {
        self.token_handler.next();
        let pat = self.pattern()?;
        Guard::Pat(pat)
      },
      ref tok if BinOp::from_sigil_token(tok).is_some() => {
        let op = BinOp::from_sigil_token(&self.token_handler.next().kind).unwrap();
        let precedence = op.get_precedence();
        let Expression { kind, .. } = self.precedence_expr(precedence)?;
        Guard::HalfExpr(HalfExpr { op: Some(op), expr: kind })
      },
      _ => {
        //TODO - I think there's a better way to do this involving the precedence of ->
        let Expression { kind, .. } = self.prefix_expr()?;
        Guard::HalfExpr(HalfExpr { op: None, expr: kind })
      }
    })
  }

  #[recursive_descent_method]
  fn pattern(&mut self) -> ParseResult<Pattern> {
    if let LParen = self.token_handler.peek_kind() {
      let tuple_pattern_variants = delimited!(self, LParen, pattern, Comma, RParen);
      Ok(Pattern::TuplePattern(tuple_pattern_variants))
    } else {
      self.simple_pattern()
    }
  }

  #[recursive_descent_method]
  fn simple_pattern(&mut self) -> ParseResult<Pattern> {
    Ok(match self.token_handler.peek_kind() {
      Identifier(_) => {
        let components = self.qualified_identifier()?;
        let qualified_identifier = QualifiedName { id: self.id_store.fresh(), components };
        match self.token_handler.peek_kind() {
          LCurlyBrace => {
            let members = delimited!(self, LCurlyBrace, record_pattern_entry, Comma, RCurlyBrace);
            Pattern::Record(qualified_identifier, members)
          },
          LParen => {
            let members = delimited!(self, LParen, pattern, Comma, RParen);
            Pattern::TupleStruct(qualified_identifier, members)
          },
          _ => {
            Pattern::VarOrName(qualified_identifier)
          },
        }
      },
      _ => self.pattern_literal()?
    })
  }

  #[recursive_descent_method]
  fn pattern_literal(&mut self) -> ParseResult<Pattern> {
    let tok = self.token_handler.peek();
    Ok(match tok.get_kind() {
      Keyword(Kw::True) => {
        self.token_handler.next();
        Pattern::Literal(PatternLiteral::BoolPattern(true))
      },
      Keyword(Kw::False) => {
        self.token_handler.next();
        Pattern::Literal(PatternLiteral::BoolPattern(false))
      },
      StrLiteral(s) => {
        self.token_handler.next();
        Pattern::Literal(PatternLiteral::StringPattern(s))
      },
      DigitGroup(_) | HexLiteral(_) | BinNumberSigil | Period => self.signed_number_literal()?,
      Operator(ref op) if **op == "-" => self.signed_number_literal()?,
      Underscore => {
        self.token_handler.next();
        Pattern::Ignored
      },
      other => return ParseError::new_with_token(format!("{:?} is not a valid Pattern", other), tok)
    })
  }

  /*
  #[recursive_descent_method]
  fn simple_pattern(&mut self) -> ParseResult<Pattern> {
    Ok({
      let tok = self.token_handler.peek();
      match tok.get_kind() {
        Identifier(_) => {
          let id = self.identifier()?;
          match self.token_handler.peek_kind() {
            LCurlyBrace => {
              let members = delimited!(self, LCurlyBrace, record_pattern_entry, Comma, RCurlyBrace);
              Pattern::Record(id, members)
            },
            LParen => {
              let members = delimited!(self, LParen, pattern, Comma, RParen);
              Pattern::TupleStruct(id, members)
            },
            _ => Pattern::Literal(PatternLiteral::VarPattern(id))
          }
        },
        Keyword(Kw::True) => {
          self.token_handler.next();
          Pattern::Literal(PatternLiteral::BoolPattern(true))
        },
        Keyword(Kw::False) => {
          self.token_handler.next();
          Pattern::Literal(PatternLiteral::BoolPattern(false))
        },
        StrLiteral(s) => {
          self.token_handler.next();
          Pattern::Literal(PatternLiteral::StringPattern(s))
        },
        DigitGroup(_) | HexLiteral(_) | BinNumberSigil | Period => self.signed_number_literal()?,
        Operator(ref op) if **op == "-" => self.signed_number_literal()?,
        Underscore => {
          self.token_handler.next();
          Pattern::Ignored
        },
        other => return ParseError::new_with_token(format!("{:?} is not a valid Pattern", other), tok)
      }
    })
  }
  */

  #[recursive_descent_method]
  fn signed_number_literal(&mut self) -> ParseResult<Pattern> {
    let neg = match self.token_handler.peek_kind() {
      Operator(ref op) if **op == "-" => {
        self.token_handler.next();
        true
      },
      _ => false
    };
    let Expression { kind, .. } = self.number_literal()?;
    Ok(Pattern::Literal(PatternLiteral::NumPattern { neg, num: kind }))
  }

  #[recursive_descent_method]
  fn record_pattern_entry(&mut self) -> ParseResult<(Rc<String>, Pattern)> {
    let name = self.identifier()?;
    Ok(match self.token_handler.peek_kind() {
      Colon => {
        expect!(self, Colon);
        let pat = self.pattern()?;
        (name, pat)
      },
      _ => (name.clone(), Pattern::Literal(PatternLiteral::StringPattern(name.clone())))
    })
  }

  #[recursive_descent_method]
  fn block(&mut self) -> ParseResult<Block> {
    let block = delimited!(self, LCurlyBrace, statement, Newline | Semicolon, RCurlyBrace, nonstrict);
    Ok(block)
  }

  #[recursive_descent_method]
  fn expr_or_block(&mut self) -> ParseResult<Block> {
    match self.token_handler.peek_kind() {
      LCurlyBrace => self.block(),
      _ => {
        let expr = self.expression()?;
        let s = Statement { id: self.id_store.fresh(), kind: StatementKind::Expression(expr) };
        Ok(vec![s])
      }
    }
  }

  #[recursive_descent_method]
  fn while_expr(&mut self) -> ParseResult<Expression> {
    use self::ExpressionKind::*;
    expect!(self, Keyword(Kw::While));
    let condition =  {
      self.restrictions.no_struct_literal = true;
      let x = self.while_cond();
      self.restrictions.no_struct_literal = false;
      x?.map(|expr| bx!(expr))
    };
    let body = self.block()?;
    Ok(Expression::new(self.id_store.fresh(), WhileExpression {condition, body}))
  }

  #[recursive_descent_method]
  fn while_cond(&mut self) -> ParseResult<Option<Expression>> {
    Ok(match self.token_handler.peek_kind() {
      LCurlyBrace => None,
      _ => Some(self.expression()?),
    })
  }

  #[recursive_descent_method]
  fn for_expr(&mut self) -> ParseResult<Expression> {
    expect!(self, Keyword(Kw::For));
    let enumerators = if let LCurlyBrace = self.token_handler.peek_kind() {
      delimited!(self, LCurlyBrace, enumerator, Comma | Newline, RCurlyBrace)
    } else {
      let single_enum = {
        self.restrictions.no_struct_literal = true;
        let s = self.enumerator();
        self.restrictions.no_struct_literal = false;
        s?
      };
      vec![single_enum]
    };
    let body = Box::new(self.for_expr_body()?);
    Ok(Expression::new(self.id_store.fresh(), ExpressionKind::ForExpression { enumerators, body }))
  }

  #[recursive_descent_method]
  fn enumerator(&mut self) -> ParseResult<Enumerator> {
    let id = self.identifier()?;
    expect!(self, Operator(ref c) if **c == "<-");
    let generator = self.expression()?;
    Ok(Enumerator { id, generator })
  }

  #[recursive_descent_method]
  fn for_expr_body(&mut self) -> ParseResult<ForBody> {
    use self::ForBody::*;
    let tok = self.token_handler.peek();
    Ok(match tok.get_kind() {
      LCurlyBrace => {
        let statements = delimited!(self, LCurlyBrace, statement, Newline | Semicolon, RCurlyBrace, nonstrict);
        StatementBlock(statements)
      },
      Keyword(Kw::Return) => {
        self.token_handler.next();
        MonadicReturn(self.expression()?)
      },
      _ => return ParseError::new_with_token("for expressions must end in a block or 'return'", tok),
    })
  }

  #[recursive_descent_method]
  fn identifier(&mut self) -> ParseResult<Rc<String>> {
    let tok = self.token_handler.next();
    match tok.get_kind() {
      Identifier(s) => Ok(s),
      p => ParseError::new_with_token(format!("Expected an identifier, got {:?}", p), tok),
    }
  }

  #[recursive_descent_method]
  fn literal(&mut self) -> ParseResult<Expression> {
    use self::ExpressionKind::*;

    let tok = self.token_handler.peek();
    match tok.get_kind() {
      DigitGroup(_) | HexLiteral(_) | BinNumberSigil | Period => self.number_literal(),
      Keyword(Kw::True) => {
        self.token_handler.next();
        let id = self.id_store.fresh();
        Ok(Expression::new(id, BoolLiteral(true)))
      },
      Keyword(Kw::False) => {
        self.token_handler.next();
        let id = self.id_store.fresh();
        Ok(Expression::new(id, BoolLiteral(false)))
      },
      StrLiteral(s) => {
        self.token_handler.next();
        let id = self.id_store.fresh();
        Ok(Expression::new(id, StringLiteral(s.clone())))
      }
      e => ParseError::new_with_token(format!("Expected a literal expression, got {:?}", e), tok),
    }
  }

  #[recursive_descent_method]
  fn number_literal(&mut self) -> ParseResult<Expression> {
    match self.token_handler.peek_kind() {
      HexLiteral(_) | BinNumberSigil => self.int_literal(),
      _ => self.float_literal(),
    }
  }

  #[recursive_descent_method]
  fn int_literal(&mut self) -> ParseResult<Expression> {
    use self::ExpressionKind::*;
    let tok = self.token_handler.next();
    match tok.get_kind() {
      BinNumberSigil => {
        let digits = self.digits()?;
        let n = parse_binary(digits, tok)?;
        Ok(Expression::new(self.id_store.fresh(), NatLiteral(n)))
      },
      HexLiteral(text) => {
        let digits: String = text.chars().filter(|c| c.is_digit(16)).collect();
        let n = parse_hex(digits, tok)?;
        Ok(Expression::new(self.id_store.fresh(), NatLiteral(n)))
      },
      _ => return ParseError::new_with_token("Expected '0x' or '0b'", tok),
    }
  }

  #[recursive_descent_method]
  fn float_literal(&mut self) -> ParseResult<Expression> {
    use self::ExpressionKind::*;
    let tok = self.token_handler.peek();
    let mut digits = self.digits()?;
    if let Period = self.token_handler.peek_kind() {
      self.token_handler.next();
      digits.push_str(".");
      digits.push_str(&self.digits()?);
      match digits.parse::<f64>() {
        Ok(f) => Ok(Expression::new(self.id_store.fresh(), FloatLiteral(f))),
        Err(e) => ParseError::new_with_token(format!("Float failed to parse with error: {}", e), tok),

      }
    } else {
      match digits.parse::<u64>() {
        Ok(d) => Ok(Expression::new(self.id_store.fresh(), NatLiteral(d))),
        Err(e) => ParseError::new_with_token(format!("Integer failed to parse with error: {}", e), tok),
      }
    }
  }

  #[recursive_descent_method]
  fn digits(&mut self) -> ParseResult<String> {
    let mut ds = String::new();
    loop {
      match self.token_handler.peek_kind() {
        Underscore => { self.token_handler.next(); continue; },
        DigitGroup(ref s) => { self.token_handler.next(); ds.push_str(s)},
        _ => break,
      }
    }
    Ok(ds)
  }

  #[recursive_descent_method]
  fn import_declaration(&mut self) -> ParseResult<ImportSpecifier> {
    expect!(self, Keyword(Import));
    let mut path_components = vec![];
    path_components.push(self.identifier()?);
    loop {
      match (self.token_handler.peek_kind(), self.token_handler.peek_kind_n(1)) {
        (Colon, Colon) => {
          self.token_handler.next(); self.token_handler.next();
          if let Identifier(_) = self.token_handler.peek_kind() {
            path_components.push(self.identifier()?);
          } else {
            break;
          }
        },
        _ => break,
      }
    }

    let imported_names = match self.token_handler.peek_kind() {
      LCurlyBrace => {
        let names = delimited!(self, LCurlyBrace, identifier, Comma, RCurlyBrace);
        ImportedNames::List(names)
      },
      Operator(ref s) if **s == "*" => {
        self.token_handler.next();
        ImportedNames::All
      },
      _ => ImportedNames::LastOfPath
    };

    Ok(ImportSpecifier {
      id: self.id_store.fresh(),
      path_components,
      imported_names
    })
  }

  #[recursive_descent_method]
  fn import_suffix(&mut self) -> ParseResult<ImportedNames> {
    Ok(match self.token_handler.peek_kind() {
      Operator(ref s) if **s == "*" => {
        self.token_handler.next();
        ImportedNames::All
      },
      LCurlyBrace => {
        let names = delimited!(self, LCurlyBrace, identifier, Comma, RCurlyBrace);
        ImportedNames::List(names)
      },
      _ => return ParseError::new_with_token("Expected '{{' or '*'", self.token_handler.peek()),
    })
  }
}

fn parse_binary(digits: String, tok: Token) -> ParseResult<u64> {
  let mut result: u64 = 0;
  let mut multiplier = 1;
  for d in digits.chars().rev() {
    match d {
      '1' => result += multiplier,
      '0' => (),
      _ => return ParseError::new_with_token("Encountered a character not '1' or '0 while parsing a binary literal", tok),
    }
    multiplier = match multiplier.checked_mul(2) {
      Some(m) => m,
      None => return ParseError::new_with_token("This binary expression will overflow", tok)
    }
  }
  Ok(result)
}

fn parse_hex(digits: String, tok: Token) -> ParseResult<u64> {
  let mut result: u64 = 0;
  let mut multiplier: u64 = 1;
  for d in digits.chars().rev() {
    match d.to_digit(16) {
      Some(n) => result += n as u64 * multiplier,
      None => return ParseError::new_with_token("Encountered a non-hex digit in a hex literal", tok),
    }
    multiplier = match multiplier.checked_mul(16) {
      Some(m) => m,
      None => return ParseError::new_with_token("This hex expression will overflow", tok)
    }
  }
  Ok(result)
}