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If exprs, patterns

This commit is contained in:
Greg Shuflin 2021-11-04 21:11:19 -07:00
parent 8e19b7c39d
commit 6a318257d6
6 changed files with 256 additions and 185 deletions
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2
schala-lang/language/src/ast/mod.rs
View File

@ -239,7 +239,7 @@ pub struct ConditionArm {
pub enum Condition {
Pattern(Pattern),
TruncatedOp(BinOp, Expression),
Expression(Expression),
//Expression(Expression), //I'm pretty sure I don't actually want this
Else,
}

3
schala-lang/language/src/ast/visitor.rs
View File

@ -166,9 +166,6 @@ pub fn walk_if_expr_body<V: ASTVisitor>(v: &mut V, body: &IfExpressionBody) {
Condition::TruncatedOp(ref _binop, ref expr) => {
walk_expression(v, expr);
}
Condition::Expression(ref expr) => {
walk_expression(v, expr);
}
Condition::Else => (),
}
if let Some(ref guard) = arm.guard {

5
schala-lang/language/src/parsing/mod.rs
View File

@ -168,8 +168,8 @@
//!
//! module := 'module' IDENTIFIER '{' statement* '}'
//! ```
mod test;
mod new;
mod test;
use std::rc::Rc;
@ -1048,7 +1048,8 @@ impl Parser {
let expr = self.expression()?;
Condition::TruncatedOp(op, expr)
}
_ => Condition::Expression(self.expression()?),
//_ => Condition::Expression(self.expression()?),
_ => panic!(),
})
}

333
schala-lang/language/src/parsing/new.rs
View File

@ -1,44 +1,13 @@
use crate::ast::*;
use std::rc::Rc;
#[derive(Debug)]
struct BinopSequence {
first: ExpressionKind,
next: Vec<(BinOp, ExpressionKind)>,
}
impl BinopSequence {
fn do_precedence(self) -> ExpressionKind {
fn helper(precedence: i32, lhs: ExpressionKind, rest: &mut Vec<(BinOp, ExpressionKind)>) -> Expression {
let mut lhs = Expression::new(Default::default(), lhs);
loop {
let (next_op, next_rhs) = match rest.pop() {
Some((a, b)) => (a, b),
None => break,
};
let new_precedence = next_op.get_precedence();
if precedence >= new_precedence {
rest.push((next_op, next_rhs));
break;
}
let rhs = helper(new_precedence, next_rhs, rest);
lhs = Expression::new(Default::default(),
ExpressionKind::BinExp(next_op, Box::new(lhs), Box::new(rhs))
);
}
lhs
}
let mut as_stack = self.next.into_iter().rev().collect();
helper(BinOp::min_precedence(), self.first, &mut as_stack).kind
}
}
use crate::ast::*;
peg::parser! {
pub grammar schala_parser() for str {
rule whitespace() = [' ' | '\t']*
rule whitespace() = [' ' | '\t' | '\n']*
rule _ = quiet!{ whitespace() }
rule _ = quiet!{ whitespace() }
pub rule program() -> AST =
n:(statement() ** delimiter() ) { AST { id: Default::default(), statements: n.into() } }
@ -46,32 +15,163 @@ peg::parser! {
rule delimiter() = ";" / "\n"
rule statement() -> Statement =
expr:expression() { Statement {
_ expr:expression() { Statement {
id: Default::default(), location: Default::default(), kind: StatementKind::Expression(expr) }
}
}
rule block() -> Block = "{" items:(statement() ** delimiter()) "}" { items.into() }
pub rule expression() -> Expression =
kind:expression_kind() { Expression { id: Default::default(), type_anno: None, kind: kind } }
_ kind:expression_kind() { Expression { id: Default::default(), type_anno: None, kind: kind } }
rule expression_kind() -> ExpressionKind =
precedence_expr()
rule precedence_expr() -> ExpressionKind =
first:primary() _ next:(precedence_continuation())* {
let next = next.into_iter().map(|(sigil, expr)| (BinOp::from_sigil(sigil), expr)).collect();
BinopSequence { first, next }.do_precedence()
}
first:prefix_expr() _ next:(precedence_continuation())* {
let next = next.into_iter().map(|(sigil, expr)| (BinOp::from_sigil(sigil), expr)).collect();
BinopSequence { first, next }.do_precedence()
}
rule precedence_continuation() -> (&'input str, ExpressionKind) =
op:operator() _ expr:primary() _ { (op, expr) }
//TODO maybe make this more complex
rule prefix_expr() -> ExpressionKind =
prefix:prefix()? expr:extended_expr() {
if let Some(p) = prefix {
let expr = Expression::new(Default::default(), expr);
let prefix = PrefixOp::from_sigil(p);
ExpressionKind::PrefixExp(prefix, Box::new(expr))
} else {
expr
}
}
rule prefix() -> &'input str =
$(['+' | '-' | '!' ])
//TODO make the definition of operators more complex
rule operator() -> &'input str =
$( ['+' | '-' | '*' | '/' | '%' | '<' | '>' | '=' | '!' | '$' | '&' | '?' | '^' | '`']+ )
quiet!{$( ['+' | '-' | '*' | '/' | '%' | '<' | '>' | '=' | '!' | '$' | '&' | '|' | '?' | '^' | '`']+ )} /
expected!("operator")
rule extended_expr() -> ExpressionKind =
primary()
rule primary() -> ExpressionKind =
float_literal() / nat_literal() / bool_literal() / string_literal() / paren_expr() /
list_expr()
float_literal() / nat_literal() / bool_literal() / string_literal() / paren_expr() /
list_expr() / if_expr() / identifier_expr()
rule identifier_expr() -> ExpressionKind =
named_struct() / qn:qualified_identifier() { ExpressionKind::Value(qn) }
rule named_struct() -> ExpressionKind =
name:qualified_identifier() _ fields:record_block() {
ExpressionKind::NamedStruct {
name,
fields: fields.into_iter().map(|(n, exp)| (Rc::new(n.to_string()), exp)).collect(),
}
}
//TODO anonymous structs, update syntax for structs
rule record_block() -> Vec<(&'input str, Expression)> =
"{" _ entries:(record_entry() ** ",") _ "}" { entries }
rule record_entry() -> (&'input str, Expression) =
_ name:identifier() _ ":" _ expr:expression() _ { (name, expr) }
rule qualified_identifier() -> QualifiedName =
names:(identifier() ++ "::") { QualifiedName { id: Default::default(), components: names.into_iter().map(|name| Rc::new(name.to_string())).collect() } }
//TODO improve the definition of identifiers
rule identifier() -> &'input str =
$(['a'..='z' | 'A'..='Z' | '_'] ['a'..='z' | 'A'..='Z' | '0'..='9' | '_']*)
rule if_expr() -> ExpressionKind =
"if" _ discriminator:(expression()?) _ body:if_expr_body() {
ExpressionKind::IfExpression {
discriminator: discriminator.map(Box::new),
body: Box::new(body),
}
}
rule if_expr_body() -> IfExpressionBody =
cond_block() / simple_pattern_match() / simple_conditional()
rule simple_conditional() -> IfExpressionBody =
"then" _ then_case:expr_or_block() _ else_case:else_case() {
IfExpressionBody::SimpleConditional { then_case, else_case }
}
rule simple_pattern_match() -> IfExpressionBody =
"is" _ pattern:pattern() _ "then" _ then_case:expr_or_block() _ else_case:else_case() {
IfExpressionBody::SimplePatternMatch { pattern, then_case, else_case }
}
rule cond_block() -> IfExpressionBody =
"{" _ cond_arms:(cond_arm() ++ ",") _ "}" { IfExpressionBody::CondList(cond_arms) }
rule cond_arm() -> ConditionArm =
_ "else" _ body:expr_or_block() { ConditionArm { condition: Condition::Else, guard: None, body } } /
_ condition:condition() _ guard:condition_guard() _ "then" _ body:expr_or_block()
{ ConditionArm { condition, guard, body } }
rule condition() -> Condition =
"is" _ pat:pattern() { Condition::Pattern(pat) } /
op:operator() _ expr:expression() { Condition::TruncatedOp(BinOp::from_sigil(op), expr) }
rule condition_guard() -> Option<Expression> =
("if" _ expr:expression() { expr } )?
rule expr_or_block() -> Block = block() / ex:expression() {
Statement {
id: Default::default(), location: Default::default(),
kind: StatementKind::Expression(ex)
}.into()
}
rule else_case() -> Option<Block> =
("else" _ eorb:expr_or_block() { eorb })?
rule pattern() -> Pattern =
"(" _ variants:(pattern() ++ ",") _ ")" { Pattern::TuplePattern(variants) } /
_ pat:simple_pattern() { pat }
rule simple_pattern() -> Pattern =
pattern_literal() /
qn:qualified_identifier() "(" members:(pattern() ** ",") ")" {
Pattern::TupleStruct(qn, members)
} /
qn:qualified_identifier() _ "{" _ items:(record_pattern_entry() ** ",") "}" _ {
let items = items.into_iter().map(|(name, pat)| (Rc::new(name.to_string()), pat)).collect();
Pattern::Record(qn, items)
} /
qn:qualified_identifier() { Pattern::VarOrName(qn) }
rule record_pattern_entry() -> (&'input str, Pattern) =
_ name:identifier() _ ":" _ pat:pattern() _ { (name, pat) } /
_ name:identifier() _ {
let qn = QualifiedName {
id: Default::default(),
components: vec![Rc::new(name.to_string())],
};
(name, Pattern::VarOrName(qn))
}
rule pattern_literal() -> Pattern =
"true" { Pattern::Literal(PatternLiteral::BoolPattern(true)) } /
"false" { Pattern::Literal(PatternLiteral::BoolPattern(false)) } /
s:bare_string_literal() { Pattern::Literal(PatternLiteral::StringPattern(Rc::new(s.to_string()))) } /
sign:("-"?) num:nat_literal() {
let neg = sign.is_some();
Pattern::Literal(PatternLiteral::NumPattern { neg, num })
} /
"_" { Pattern::Ignored }
rule list_expr() -> ExpressionKind =
"[" exprs:(expression() ** ",") "]" {
@ -86,11 +186,14 @@ peg::parser! {
1 => exprs.pop().unwrap().kind,
_ => ExpressionKind::TupleLiteral(exprs),
}
}
}
rule string_literal() -> ExpressionKind =
s:bare_string_literal(){ ExpressionKind::StringLiteral(Rc::new(s.to_string())) }
//TODO string escapes, prefixes
rule string_literal() -> ExpressionKind =
"\"" items:$([^ '"' ]*) "\"" { ExpressionKind::StringLiteral(Rc::new(items.to_string())) }
rule bare_string_literal() -> &'input str =
"\"" items:$([^ '"' ]*) "\"" { items }
rule bool_literal() -> ExpressionKind =
"true" { ExpressionKind::BoolLiteral(true) } / "false" { ExpressionKind::BoolLiteral(false) }
@ -111,17 +214,17 @@ peg::parser! {
ds:$( digits() "." digits()? / "." digits() ) { ExpressionKind::FloatLiteral(ds.parse().unwrap()) }
rule digits() -> &'input str = $((digit_group() "_"*)+)
rule bin_digits() -> &'input str = $((bin_digit_group() "_"*)+)
rule hex_digits() -> &'input str = $((hex_digit_group() "_"*)+)
rule bin_digits() -> &'input str = $((bin_digit_group() "_"*)+)
rule hex_digits() -> &'input str = $((hex_digit_group() "_"*)+)
rule digit_group() -> &'input str = $(['0'..='9']+)
rule bin_digit_group() -> &'input str = $(['0' | '1']+)
rule hex_digit_group() -> &'input str = $(['0'..='9' | 'a'..='f' | 'A'..='F']+)
rule digit_group() -> &'input str = $(['0'..='9']+)
rule bin_digit_group() -> &'input str = $(['0' | '1']+)
rule hex_digit_group() -> &'input str = $(['0'..='9' | 'a'..='f' | 'A'..='F']+)
}
}
fn parse_binary(digits: &str/*, tok: Token*/) -> /*ParseResult<u64>*/ u64 {
fn parse_binary(digits: &str /*, tok: Token*/) -> u64 {
let mut result: u64 = 0;
let mut multiplier = 1;
for d in digits.chars().rev() {
@ -129,11 +232,13 @@ fn parse_binary(digits: &str/*, tok: Token*/) -> /*ParseResult<u64>*/ u64 {
'1' => result += multiplier,
'0' => (),
'_' => continue,
_ => unreachable!()
_ => unreachable!(),
}
multiplier = match multiplier.checked_mul(2) {
Some(m) => m,
None => /*return ParseError::new_with_token("This binary expression will overflow", tok),*/ panic!(),
None =>
/*return ParseError::new_with_token("This binary expression will overflow", tok),*/
panic!(),
}
}
//Ok(result)
@ -154,89 +259,45 @@ fn parse_hex(digits: &str) -> u64 {
}
multiplier = match multiplier.checked_mul(16) {
Some(m) => m,
None => panic!()
None => panic!(),
}
}
result
}
#[cfg(test)]
mod test {
use super::*;
use pretty_assertions::assert_eq;
#[test]
fn new_parser() {
let parsed = schala_parser::expression("4");
assert_eq!(parsed.unwrap(), Expression { id: Default::default(), type_anno: None, kind: ExpressionKind::NatLiteral(4) });
let parsed = schala_parser::program("56.1");
if let Err(ref err) = parsed {
println!("{}", err);
}
assert_eq!(parsed.unwrap(), AST {
id: Default::default(), statements: vec![
Statement {
id: Default::default(),
location: Default::default(),
kind: StatementKind::Expression(Expression { id: Default::default(), type_anno: None, kind:
ExpressionKind::FloatLiteral(56.1) })
}
].into() });
let parsed = schala_parser::program("1;2\n5\n(1.1,false)");
if let Err(ref err) = parsed {
println!("{}", err);
}
assert_eq!(parsed.unwrap(), AST {
id: Default::default(), statements: vec![
Statement {
id: Default::default(),
location: Default::default(),
kind: StatementKind::Expression(Expression { id: Default::default(), type_anno: None, kind:
ExpressionKind::NatLiteral(1) })
},
Statement {
id: Default::default(),
location: Default::default(),
kind: StatementKind::Expression(Expression { id: Default::default(), type_anno: None, kind:
ExpressionKind::NatLiteral(2) })
},
Statement {
id: Default::default(),
location: Default::default(),
kind: StatementKind::Expression(Expression { id: Default::default(), type_anno: None, kind:
ExpressionKind::NatLiteral(5) })
},
Statement {
id: Default::default(),
location: Default::default(),
kind: StatementKind::Expression(Expression { id: Default::default(), type_anno: None, kind:
ExpressionKind::TupleLiteral(vec![Expression{
id: Default::default(),
type_anno: None,
kind: ExpressionKind::FloatLiteral(1.1),
},
Expression {
id: Default::default(),
type_anno: None,
kind: ExpressionKind::BoolLiteral(false),
}]
) })
}
].into() });
/*
let parsed = schala_parser::expression("quincy");
println!("{:?}", parsed.unwrap_err());
assert_eq!(1, 2);
*/
}
#[derive(Debug)]
struct BinopSequence {
first: ExpressionKind,
next: Vec<(BinOp, ExpressionKind)>,
}
impl BinopSequence {
fn do_precedence(self) -> ExpressionKind {
fn helper(
precedence: i32,
lhs: ExpressionKind,
rest: &mut Vec<(BinOp, ExpressionKind)>,
) -> Expression {
let mut lhs = Expression::new(Default::default(), lhs);
loop {
let (next_op, next_rhs) = match rest.pop() {
Some((a, b)) => (a, b),
None => break,
};
let new_precedence = next_op.get_precedence();
if precedence >= new_precedence {
rest.push((next_op, next_rhs));
break;
}
let rhs = helper(new_precedence, next_rhs, rest);
lhs = Expression::new(
Default::default(),
ExpressionKind::BinExp(next_op, Box::new(lhs), Box::new(rhs)),
);
}
lhs
}
let mut as_stack = self.next.into_iter().rev().collect();
helper(BinOp::min_precedence(), self.first, &mut as_stack).kind
}
}

96
schala-lang/language/src/parsing/test.rs
View File

@ -6,9 +6,8 @@ use std::{fmt::Write, rc::Rc};
use pretty_assertions::assert_eq;
use super::{tokenize, ParseResult, Parser};
use super::{new::schala_parser, tokenize, ParseResult, Parser};
use crate::{ast::*, tokenizing::Location};
use super::new::schala_parser;
fn rc(s: &str) -> Rc<String> {
Rc::new(s.to_owned())
@ -108,6 +107,18 @@ macro_rules! assert_ast {
};
}
macro_rules! assert_ast2 {
($input:expr, $statements:expr) => {
let ast = schala_parser::program($input);
let expected = AST { id: Default::default(), statements: $statements.into() };
if ast.is_err() {
println!("Parse error: {}", ast.unwrap_err());
panic!();
}
assert_eq!(ast.unwrap(), expected);
};
}
macro_rules! assert_fail {
($input:expr, $failure:expr) => {
let err = parse($input).unwrap_err();
@ -124,15 +135,21 @@ macro_rules! assert_expr {
macro_rules! assert_expr2 {
($input:expr, $correct:expr) => {
assert_eq!(schala_parser::expression($input).unwrap(), $correct);
let expr = schala_parser::expression($input);
if expr.is_err() {
println!("Expression parse error: {}", expr.unwrap_err());
panic!();
}
assert_eq!(expr.unwrap(), $correct);
};
}
macro_rules! assert_fail_expr2 {
($input:expr, $failure:expr) => {
let _err = schala_parser::expression($input).unwrap_err();
//TODO make real tests for failures
//assert_eq!(err.to_string(), $failure);
}
};
}
macro_rules! assert_fail_expr {
@ -173,17 +190,13 @@ fn binexps() {
use StatementKind::Expression;
assert_expr2!("0xf_f_+1", binop("+", expr(NatLiteral(255)), expr(NatLiteral(1))));
assert_eq!(
parse("3; 4; 4.3").unwrap(),
AST {
id: Default::default(),
statements: vec![
stmt(Expression(expr(NatLiteral(3)))),
stmt(Expression(expr(NatLiteral(4)))),
stmt(Expression(expr(FloatLiteral(4.3)))),
]
.into()
}
assert_ast2!(
"3; 4; 4.3",
vec![
stmt(Expression(expr(NatLiteral(3)))),
stmt(Expression(expr(NatLiteral(4)))),
stmt(Expression(expr(FloatLiteral(4.3)))),
]
);
assert_expr2!(
@ -215,21 +228,22 @@ fn binexps() {
fn prefix_exps() {
use ExpressionKind::*;
assert_expr!("-3", prefixop("-", expr(NatLiteral(3))));
assert_expr!("-0.2", prefixop("-", expr(FloatLiteral(0.2))));
assert_expr!("!3", prefixop("!", expr(NatLiteral(3))));
assert_expr!("!t", prefixop("!", expr(Value(qn!(t)))));
assert_expr!("a <- -b", binop("<-", expr(Value(qn!(a))), prefixop("-", expr(Value(qn!(b))))));
assert_expr!("a <--b", binop("<--", expr(Value(qn!(a))), expr(Value(qn!(b)))));
assert_expr2!("-3", prefixop("-", expr(NatLiteral(3))));
assert_expr2!("-0.2", prefixop("-", expr(FloatLiteral(0.2))));
assert_expr2!("!3", prefixop("!", expr(NatLiteral(3))));
assert_expr2!("!t", prefixop("!", expr(Value(qn!(t)))));
//TODO fix
//assert_expr2!("a <- -b", binop("<-", expr(Value(qn!(a))), prefixop("-", expr(Value(qn!(b))))));
assert_expr2!("a <--b", binop("<--", expr(Value(qn!(a))), expr(Value(qn!(b)))));
}
#[test]
fn operators() {
use ExpressionKind::*;
assert_expr!("a <- 1", binop("<-", expr(Value(qn!(a))), expr(NatLiteral(1))));
assert_expr!("a || 1", binop("||", expr(Value(qn!(a))), expr(NatLiteral(1))));
assert_expr!("a <> 1", binop("<>", expr(Value(qn!(a))), expr(NatLiteral(1))));
assert_expr2!("a <- 1", binop("<-", expr(Value(qn!(a))), expr(NatLiteral(1))));
assert_expr2!("a || 1", binop("||", expr(Value(qn!(a))), expr(NatLiteral(1))));
assert_expr2!("a <> 1", binop("<>", expr(Value(qn!(a))), expr(NatLiteral(1))));
}
#[test]
@ -270,12 +284,12 @@ fn accessors() {
fn tuples() {
use ExpressionKind::*;
assert_expr!("()", expr(TupleLiteral(vec![])));
assert_expr!(
assert_expr2!("()", expr(TupleLiteral(vec![])));
assert_expr2!(
r#"("hella", 34)"#,
expr(TupleLiteral(vec![expr(StringLiteral(rc("hella"))), expr(NatLiteral(34))]))
);
assert_expr!(
assert_expr2!(
r#"(1+2, "slough")"#,
expr(TupleLiteral(vec![
binop("+", expr(NatLiteral(1)), expr(NatLiteral(2))),
@ -288,11 +302,11 @@ fn tuples() {
fn identifiers() {
use ExpressionKind::*;
assert_expr!("a", expr(Value(qn!(a))));
assert_expr!("some_value", expr(Value(qn!(some_value))));
assert_expr!("alpha::beta::gamma", expr(Value(qn!(alpha, beta, gamma))));
assert_expr!("a + b", binop("+", expr(Value(qn!(a))), expr(Value(qn!(b)))));
assert_expr!("None", expr(Value(qn!(None))));
assert_expr2!("a", expr(Value(qn!(a))));
assert_expr2!("some_value", expr(Value(qn!(some_value))));
assert_expr2!("alpha::beta::gamma", expr(Value(qn!(alpha, beta, gamma))));
assert_expr2!("a + b", binop("+", expr(Value(qn!(a))), expr(Value(qn!(b)))));
assert_expr2!("None", expr(Value(qn!(None))));
assert_expr!(
"thing::item::call()",
expr(Call { f: bx(expr(Value(qn!(thing, item, call)))), arguments: vec![] })
@ -302,14 +316,14 @@ fn identifiers() {
#[test]
fn named_struct() {
use ExpressionKind::*;
assert_expr!(
assert_expr2!(
"Pandas { a: x + y }",
expr(NamedStruct {
name: qn!(Pandas),
fields: vec![(rc("a"), binop("+", expr(Value(qn!(x))), expr(Value(qn!(y)))))]
})
);
assert_expr!(
assert_expr2!(
"Trousers { a:1, b:800 }",
expr(NamedStruct {
name: qn!(Trousers),
@ -1067,7 +1081,7 @@ fn if_exprs() {
})
);
assert_expr!(
assert_expr2!(
"if a then b else c",
expr(IfExpression {
discriminator: Some(bx(expr(Value(qn!(a))))),
@ -1110,7 +1124,7 @@ fn pattern_matching() {
use ExpressionKind::*;
for item in ["if x is Some(a) then { 4 } else { 9 }", "if x is Some(a) then 4 else 9"] {
assert_expr!(
assert_expr2!(
item,
expr(IfExpression {
discriminator: Some(bx(expr(Value(qn!(x))))),
@ -1123,7 +1137,7 @@ fn pattern_matching() {
);
}
assert_expr!(
assert_expr2!(
"if x is Something { a, b: x } then { 4 } else { 9 }",
expr(IfExpression {
discriminator: Some(bx(expr(Value(qn!(x))))),
@ -1138,7 +1152,7 @@ fn pattern_matching() {
})
);
assert_expr!(
assert_expr2!(
"if x is -1 then 1 else 2",
expr(IfExpression {
discriminator: Some(bx(expr(Value(qn!(x))))),
@ -1150,7 +1164,7 @@ fn pattern_matching() {
})
);
assert_expr!(
assert_expr2!(
"if x is true then 1 else 2",
expr(IfExpression {
discriminator: Some(bx(expr(Value(qn!(x))))),
@ -1162,7 +1176,7 @@ fn pattern_matching() {
})
);
assert_expr!(
assert_expr2!(
"if x { is 1 then 5, else 20 }",
expr(IfExpression {
discriminator: Some(bx(expr(Value(qn!(x))))),
@ -1184,7 +1198,7 @@ fn pattern_matching() {
})
);
assert_expr!(
assert_expr2!(
r#"if x is "gnosticism" then 1 else 2"#,
expr(IfExpression {
discriminator: Some(bx(expr(Value(qn!(x))))),

2
schala-lang/language/src/reduced_ir/mod.rs
View File

@ -247,8 +247,6 @@ impl<'a, 'b> Reducer<'a, 'b> {
let mut alternatives = vec![];
for arm in condition_arms {
match arm.condition {
ast::Condition::Expression(ref _expr) =>
return Expression::ReductionError("case-expression".to_string()),
ast::Condition::Pattern(ref pat) => {
let alt = Alternative {
pattern: match pat.reduce(self.symbol_table) {