Improve parsing of scalar literals

* Unify parsing of string literals and scalar literals, to (e.g.) ensure escapes are handled uniformly. Notably, this makes unicode escapes valid in scalar literals.
* Add a variety of custom error messages about specific failure cases of parsing string/scalar literals. For example, if we're expecting a string (e.g. a package name in the header) and the user tried using single quotes, give a clear message about that.
* Fix formatting of unicode escapes (they previously used {}, now correctly use () to match roc strings)

crates/ast/src/lang/core/pattern.rs

@@ -3,7 +3,7 @@
 #![allow(unused_imports)]
 
 use bumpalo::collections::Vec as BumpVec;
-use roc_can::expr::{unescape_char, IntValue};
+use roc_can::expr::IntValue;
 use roc_can::num::{
     finish_parsing_base, finish_parsing_float, finish_parsing_num, ParsedNumResult,
 };
@@ -620,7 +620,7 @@ pub(crate) fn flatten_str_lines(pool: &mut Pool, lines: &[&[StrSegment<'_>]]) ->
                 Interpolated(loc_expr) => {
                     return Pattern2::UnsupportedPattern(loc_expr.region);
                 }
-                EscapedChar(escaped) => buf.push(unescape_char(escaped)),
+                EscapedChar(escaped) => buf.push(escaped.unescape()),
             }
         }
     }

crates/ast/src/lang/core/str.rs

@@ -117,7 +117,7 @@ fn flatten_str_lines<'a>(
                         todo!()
                     }
                 }
-                EscapedChar(escaped) => buf.push(roc_can::expr::unescape_char(escaped)),
+                EscapedChar(escaped) => buf.push(escaped.unescape()),
             }
         }
     }

crates/compiler/can/src/expr.rs

@@ -18,7 +18,7 @@ use roc_module::called_via::CalledVia;
 use roc_module::ident::{ForeignSymbol, Lowercase, TagName};
 use roc_module::low_level::LowLevel;
 use roc_module::symbol::Symbol;
-use roc_parse::ast::{self, Defs, EscapedChar, StrLiteral};
+use roc_parse::ast::{self, Defs, StrLiteral};
 use roc_parse::pattern::PatternType::*;
 use roc_problem::can::{PrecedenceProblem, Problem, RuntimeError};
 use roc_region::all::{Loc, Region};
@@ -2297,7 +2297,7 @@ fn flatten_str_lines<'a>(
                         );
                     }
                 }
-                EscapedChar(escaped) => buf.push(unescape_char(escaped)),
+                EscapedChar(escaped) => buf.push(escaped.unescape()),
             }
         }
     }
@@ -2355,19 +2355,6 @@ fn desugar_str_segments(var_store: &mut VarStore, segments: Vec<StrSegment>) ->
     loc_expr.value
 }
 
-/// Returns the char that would have been originally parsed to
-pub fn unescape_char(escaped: &EscapedChar) -> char {
-    use EscapedChar::*;
-
-    match escaped {
-        Backslash => '\\',
-        Quote => '"',
-        CarriageReturn => '\r',
-        Tab => '\t',
-        Newline => '\n',
-    }
-}
-
 #[derive(Clone, Debug)]
 pub struct Declarations {
     pub declarations: Vec<DeclarationTag>,

crates/compiler/can/src/pattern.rs

@@ -1,6 +1,6 @@
 use crate::annotation::freshen_opaque_def;
 use crate::env::Env;
-use crate::expr::{canonicalize_expr, unescape_char, Expr, IntValue, Output};
+use crate::expr::{canonicalize_expr, Expr, IntValue, Output};
 use crate::num::{
     finish_parsing_base, finish_parsing_float, finish_parsing_num, FloatBound, IntBound, NumBound,
     ParsedNumResult,
@@ -935,7 +935,7 @@ fn flatten_str_lines(lines: &[&[StrSegment<'_>]]) -> Pattern {
                 Interpolated(loc_expr) => {
                     return Pattern::UnsupportedPattern(loc_expr.region);
                 }
-                EscapedChar(escaped) => buf.push(unescape_char(escaped)),
+                EscapedChar(escaped) => buf.push(escaped.unescape()),
             }
         }
     }

crates/compiler/fmt/src/expr.rs

@@ -431,14 +431,30 @@ impl<'a> Formattable for Expr<'a> {
     }
 }
 
+fn needs_unicode_escape(ch: char) -> bool {
+    matches!(ch, '\u{0000}'..='\u{001f}' | '\u{007f}'..='\u{009f}')
+}
+
 pub(crate) fn format_sq_literal(buf: &mut Buf, s: &str) {
     buf.push('\'');
     for c in s.chars() {
         if c == '"' {
             buf.push_char_literal('"')
         } else {
-            for escaped in c.escape_default() {
-                buf.push_char_literal(escaped);
+            match c {
+                '"' => buf.push_str("\""),
+                '\'' => buf.push_str("\\\'"),
+                '\t' => buf.push_str("\\t"),
+                '\r' => buf.push_str("\\r"),
+                '\n' => buf.push_str("\\n"),
+                '\\' => buf.push_str("\\\\"),
+                _ => {
+                    if needs_unicode_escape(c) {
+                        buf.push_str(&format!("\\u({:x})", c as u32))
+                    } else {
+                        buf.push_char_literal(c)
+                    }
+                }
             }
         }
     }

crates/compiler/parse/src/ast.rs

@@ -2,6 +2,7 @@ use std::fmt::Debug;
 
 use crate::header::{AppHeader, HostedHeader, InterfaceHeader, PackageHeader, PlatformHeader};
 use crate::ident::Ident;
+use crate::parser::ESingleQuote;
 use bumpalo::collections::{String, Vec};
 use bumpalo::Bump;
 use roc_collections::soa::{EitherIndex, Index, Slice};
@@ -116,11 +117,20 @@ pub enum StrSegment<'a> {
     Interpolated(Loc<&'a Expr<'a>>), // e.g. (name) in "Hi, \(name)!"
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum SingleQuoteSegment<'a> {
+    Plaintext(&'a str),    // e.g. 'f'
+    Unicode(Loc<&'a str>), // e.g. '00A0' in '\u(00A0)'
+    EscapedChar(EscapedChar), // e.g. '\n'
+                           // No interpolated expressions in single-quoted strings
+}
+
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum EscapedChar {
     Newline,        // \n
     Tab,            // \t
-    Quote,          // \"
+    DoubleQuote,    // \"
+    SingleQuote,    // \'
     Backslash,      // \\
     CarriageReturn, // \r
 }
@@ -132,12 +142,71 @@ impl EscapedChar {
 
         match self {
             Backslash => '\\',
-            Quote => '"',
+            SingleQuote => '\'',
+            DoubleQuote => '"',
             CarriageReturn => 'r',
             Tab => 't',
             Newline => 'n',
         }
     }
+
+    pub fn unescape(self) -> char {
+        use EscapedChar::*;
+
+        match self {
+            Backslash => '\\',
+            SingleQuote => '\'',
+            DoubleQuote => '"',
+            CarriageReturn => '\r',
+            Tab => '\t',
+            Newline => '\n',
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum SingleQuoteLiteral<'a> {
+    /// The most common case: a plain character with no escapes
+    PlainLine(&'a str),
+    Line(&'a [SingleQuoteSegment<'a>]),
+}
+
+impl<'a> SingleQuoteLiteral<'a> {
+    pub fn to_str_in(&self, arena: &'a Bump) -> &'a str {
+        match self {
+            SingleQuoteLiteral::PlainLine(s) => s,
+            SingleQuoteLiteral::Line(segments) => {
+                let mut s = String::new_in(arena);
+                for segment in *segments {
+                    match segment {
+                        SingleQuoteSegment::Plaintext(s2) => s.push_str(s2),
+                        SingleQuoteSegment::Unicode(loc) => {
+                            let s2 = loc.value;
+                            let c = u32::from_str_radix(s2, 16).expect("Invalid unicode escape");
+                            s.push(char::from_u32(c).expect("Invalid unicode codepoint"));
+                        }
+                        SingleQuoteSegment::EscapedChar(c) => {
+                            s.push(c.unescape());
+                        }
+                    }
+                }
+                s.into_bump_str()
+            }
+        }
+    }
+}
+
+impl<'a> TryFrom<StrSegment<'a>> for SingleQuoteSegment<'a> {
+    type Error = ESingleQuote;
+
+    fn try_from(value: StrSegment<'a>) -> Result<Self, Self::Error> {
+        match value {
+            StrSegment::Plaintext(s) => Ok(SingleQuoteSegment::Plaintext(s)),
+            StrSegment::Unicode(s) => Ok(SingleQuoteSegment::Unicode(s)),
+            StrSegment::EscapedChar(s) => Ok(SingleQuoteSegment::EscapedChar(s)),
+            StrSegment::Interpolated(_) => Err(ESingleQuote::InterpolationNotAllowed),
+        }
+    }
 }
 
 #[derive(Clone, Copy, Debug, PartialEq)]

crates/compiler/parse/src/expr.rs

@@ -16,6 +16,7 @@ use crate::parser::{
 };
 use crate::pattern::{closure_param, loc_has_parser};
 use crate::state::State;
+use crate::string_literal::StrLikeLiteral;
 use crate::type_annotation;
 use bumpalo::collections::Vec;
 use bumpalo::Bump;
@@ -161,8 +162,7 @@ fn loc_term_or_underscore_or_conditional<'a>(
         loc_expr_in_parens_etc_help(),
         loc!(specialize(EExpr::If, if_expr_help(options))),
         loc!(specialize(EExpr::When, when::expr_help(options))),
-        loc!(specialize(EExpr::Str, string_literal_help())),
-        loc!(specialize(EExpr::SingleQuote, single_quote_literal_help())),
+        loc!(specialize(EExpr::Str, string_like_literal_help())),
         loc!(specialize(EExpr::Number, positive_number_literal_help())),
         loc!(specialize(EExpr::Closure, closure_help(options))),
         loc!(crash_kw()),
@@ -183,8 +183,7 @@ fn loc_term_or_underscore<'a>(
 ) -> impl Parser<'a, Loc<Expr<'a>>, EExpr<'a>> {
     one_of!(
         loc_expr_in_parens_etc_help(),
-        loc!(specialize(EExpr::Str, string_literal_help())),
-        loc!(specialize(EExpr::SingleQuote, single_quote_literal_help())),
+        loc!(specialize(EExpr::Str, string_like_literal_help())),
         loc!(specialize(EExpr::Number, positive_number_literal_help())),
         loc!(specialize(EExpr::Closure, closure_help(options))),
         loc!(underscore_expression()),
@@ -200,8 +199,7 @@ fn loc_term_or_underscore<'a>(
 fn loc_term<'a>(options: ExprParseOptions) -> impl Parser<'a, Loc<Expr<'a>>, EExpr<'a>> {
     one_of!(
         loc_expr_in_parens_etc_help(),
-        loc!(specialize(EExpr::Str, string_literal_help())),
-        loc!(specialize(EExpr::SingleQuote, single_quote_literal_help())),
+        loc!(specialize(EExpr::Str, string_like_literal_help())),
         loc!(specialize(EExpr::Number, positive_number_literal_help())),
         loc!(specialize(EExpr::Closure, closure_help(options))),
         loc!(record_literal_help()),
@@ -2584,14 +2582,16 @@ fn apply_expr_access_chain<'a>(
         })
 }
 
-fn string_literal_help<'a>() -> impl Parser<'a, Expr<'a>, EString<'a>> {
-    map!(crate::string_literal::parse(), Expr::Str)
+fn string_like_literal_help<'a>() -> impl Parser<'a, Expr<'a>, EString<'a>> {
+    map_with_arena!(
+        crate::string_literal::parse_str_like_literal(),
+        |arena, lit| match lit {
+            StrLikeLiteral::Str(s) => Expr::Str(s),
+            StrLikeLiteral::SingleQuote(s) => {
+                // TODO: preserve the original escaping
+                Expr::SingleQuote(s.to_str_in(arena))
+            }
         }
-
-fn single_quote_literal_help<'a>() -> impl Parser<'a, Expr<'a>, EString<'a>> {
-    map!(
-        crate::string_literal::parse_single_quote(),
-        Expr::SingleQuote
     )
 }
 

crates/compiler/parse/src/header.rs

@@ -331,7 +331,10 @@ pub fn package_entry<'a>() -> impl Parser<'a, Spaced<'a, PackageEntry<'a>>, EPac
 
 pub fn package_name<'a>() -> impl Parser<'a, PackageName<'a>, EPackageName<'a>> {
     then(
-        loc!(specialize(EPackageName::BadPath, string_literal::parse())),
+        loc!(specialize(
+            EPackageName::BadPath,
+            string_literal::parse_str_literal()
+        )),
         move |_arena, state, progress, text| match text.value {
             StrLiteral::PlainLine(text) => Ok((progress, PackageName(text), state)),
             StrLiteral::Line(_) => Err((progress, EPackageName::Escapes(text.region.start()))),

crates/compiler/parse/src/module.rs

@@ -181,7 +181,7 @@ fn app_header<'a>() -> impl Parser<'a, AppHeader<'a>, EHeader<'a>> {
         before_name: space0_e(EHeader::IndentStart),
         name: loc!(crate::parser::specialize(
             EHeader::AppName,
-            string_literal::parse()
+            string_literal::parse_str_literal()
         )),
         packages: optional(specialize(EHeader::Packages, packages())),
         imports: optional(specialize(EHeader::Imports, imports())),

crates/compiler/parse/src/parser.rs

@@ -355,8 +355,10 @@ pub enum EExpr<'a> {
 
     InParens(EInParens<'a>, Position),
     Record(ERecord<'a>, Position),
+
+    // SingleQuote errors are folded into the EString
     Str(EString<'a>, Position),
-    SingleQuote(EString<'a>, Position),
+
     Number(ENumber, Position),
     List(EList<'a>, Position),
 
@@ -376,13 +378,24 @@ pub enum EString<'a> {
     CodePtOpen(Position),
     CodePtEnd(Position),
 
+    InvalidSingleQuote(ESingleQuote, Position),
+
     Space(BadInputError, Position),
-    EndlessSingle(Position),
-    EndlessMulti(Position),
+    EndlessSingleLine(Position),
+    EndlessMultiLine(Position),
+    EndlessSingleQuote(Position),
     UnknownEscape(Position),
     Format(&'a EExpr<'a>, Position),
     FormatEnd(Position),
     MultilineInsufficientIndent(Position),
+    ExpectedDoubleQuoteGotSingleQuote(Position),
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ESingleQuote {
+    Empty,
+    TooLong,
+    InterpolationNotAllowed,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]

crates/compiler/parse/src/pattern.rs

@@ -8,6 +8,7 @@ use crate::parser::{
     word3, EPattern, PInParens, PList, PRecord, Parser,
 };
 use crate::state::State;
+use crate::string_literal::StrLikeLiteral;
 use bumpalo::collections::string::String;
 use bumpalo::collections::Vec;
 use bumpalo::Bump;
@@ -83,8 +84,7 @@ fn loc_pattern_help_help<'a>() -> impl Parser<'a, Loc<Pattern<'a>>, EPattern<'a>
         )),
         loc!(specialize(EPattern::List, list_pattern_help())),
         loc!(number_pattern_help()),
-        loc!(string_pattern_help()),
-        loc!(single_quote_pattern_help()),
+        loc!(string_like_pattern_help()),
     )
 }
 
@@ -177,8 +177,7 @@ fn loc_parse_tag_pattern_arg<'a>() -> impl Parser<'a, Loc<Pattern<'a>>, EPattern
             EPattern::Record,
             crate::pattern::record_pattern_help()
         )),
-        loc!(string_pattern_help()),
-        loc!(single_quote_pattern_help()),
+        loc!(string_like_pattern_help()),
         loc!(number_pattern_help())
     )
 }
@@ -238,19 +237,18 @@ fn number_pattern_help<'a>() -> impl Parser<'a, Pattern<'a>, EPattern<'a>> {
     )
 }
 
-fn string_pattern_help<'a>() -> impl Parser<'a, Pattern<'a>, EPattern<'a>> {
+fn string_like_pattern_help<'a>() -> impl Parser<'a, Pattern<'a>, EPattern<'a>> {
     specialize(
         |_, pos| EPattern::Start(pos),
-        map!(crate::string_literal::parse(), Pattern::StrLiteral),
-    )
+        map_with_arena!(
+            crate::string_literal::parse_str_like_literal(),
+            |arena, lit| match lit {
+                StrLikeLiteral::Str(s) => Pattern::StrLiteral(s),
+                StrLikeLiteral::SingleQuote(s) => {
+                    // TODO: preserve the original escaping
+                    Pattern::SingleQuote(s.to_str_in(arena))
+                }
             }
-
-fn single_quote_pattern_help<'a>() -> impl Parser<'a, Pattern<'a>, EPattern<'a>> {
-    specialize(
-        |_, pos| EPattern::Start(pos),
-        map!(
-            crate::string_literal::parse_single_quote(),
-            Pattern::SingleQuote
         ),
     )
 }

crates/compiler/parse/src/string_literal.rs

@@ -1,8 +1,9 @@
-use crate::ast::{EscapedChar, StrLiteral, StrSegment};
+use crate::ast::{EscapedChar, SingleQuoteLiteral, StrLiteral, StrSegment};
 use crate::expr;
 use crate::parser::Progress::{self, *};
 use crate::parser::{
-    allocated, loc, reset_min_indent, specialize_ref, word1, BadInputError, EString, Parser,
+    allocated, loc, reset_min_indent, specialize_ref, then, word1, BadInputError, ESingleQuote,
+    EString, Parser,
 };
 use crate::state::State;
 use bumpalo::collections::vec::Vec;
@@ -31,97 +32,6 @@ fn ascii_hex_digits<'a>() -> impl Parser<'a, &'a str, EString<'a>> {
     }
 }
 
-pub fn parse_single_quote<'a>() -> impl Parser<'a, &'a str, EString<'a>> {
-    move |arena: &'a Bump, mut state: State<'a>, _min_indent: u32| {
-        if state.consume_mut("\'") {
-            // we will be parsing a single-quote-string
-        } else {
-            return Err((NoProgress, EString::Open(state.pos())));
-        }
-
-        // Handle back slaches in byte literal
-        // - starts with a backslash and used as an escape character. ex: '\n', '\t'
-        // - single quote floating (un closed single quote) should be an error
-        match state.bytes().first() {
-            Some(b'\\') => {
-                state.advance_mut(1);
-                match state.bytes().first() {
-                    Some(&ch) => {
-                        state.advance_mut(1);
-                        if (ch == b'n' || ch == b'r' || ch == b't' || ch == b'\'' || ch == b'\\')
-                            && (state.bytes().first() == Some(&b'\''))
-                        {
-                            state.advance_mut(1);
-                            let test = match ch {
-                                b'n' => '\n',
-                                b't' => '\t',
-                                b'r' => '\r',
-                                // since we checked the current char between the single quotes we
-                                // know they are valid UTF-8, allowing us to use 'from_u32_unchecked'
-                                _ => unsafe { char::from_u32_unchecked(ch as u32) },
-                            };
-
-                            return Ok((MadeProgress, &*arena.alloc_str(&test.to_string()), state));
-                        }
-                        // invalid error, backslah escaping something we do not recognize
-                        return Err((NoProgress, EString::CodePtEnd(state.pos())));
-                    }
-                    None => {
-                        // no close quote found
-                        return Err((NoProgress, EString::CodePtEnd(state.pos())));
-                    }
-                }
-            }
-            Some(_) => {
-                // do nothing for other characters, handled below
-            }
-            None => return Err((NoProgress, EString::CodePtEnd(state.pos()))),
-        }
-
-        let mut bytes = state.bytes().iter();
-        let mut end_index = 1;
-
-        // Copy paste problem in mono
-
-        loop {
-            match bytes.next() {
-                Some(b'\'') => {
-                    break;
-                }
-                Some(_) => end_index += 1,
-                None => {
-                    return Err((NoProgress, EString::Open(state.pos())));
-                }
-            }
-        }
-
-        if end_index == 1 {
-            // no progress was made
-            // this case is a double single quote, ex: ''
-            // not supporting empty single quotes
-            return Err((NoProgress, EString::Open(state.pos())));
-        }
-
-        if end_index > (std::mem::size_of::<u32>() + 1) {
-            // bad case: too big to fit into u32
-            return Err((NoProgress, EString::Open(state.pos())));
-        }
-
-        // happy case -> we have some bytes that will fit into a u32
-        // ending up w/ a slice of bytes that we want to convert into an integer
-        let raw_bytes = &state.bytes()[0..end_index - 1];
-
-        state.advance_mut(end_index);
-        match std::str::from_utf8(raw_bytes) {
-            Ok(string) => Ok((MadeProgress, string, state)),
-            Err(_) => {
-                // invalid UTF-8
-                return Err((NoProgress, EString::CodePtEnd(state.pos())));
-            }
-        }
-    }
-}
-
 fn consume_indent(mut state: State, mut indent: u32) -> Result<State, (Progress, EString)> {
     while indent > 0 {
         match state.bytes().first() {
@@ -156,11 +66,28 @@ fn utf8<'a>(state: State<'a>, string_bytes: &'a [u8]) -> Result<&'a str, (Progre
     })
 }
 
-pub fn parse<'a>() -> impl Parser<'a, StrLiteral<'a>, EString<'a>> {
-    use StrLiteral::*;
+pub enum StrLikeLiteral<'a> {
+    SingleQuote(SingleQuoteLiteral<'a>),
+    Str(StrLiteral<'a>),
+}
 
+pub fn parse_str_literal<'a>() -> impl Parser<'a, StrLiteral<'a>, EString<'a>> {
+    then(
+        loc!(parse_str_like_literal()),
+        |_arena, state, progress, str_like| match str_like.value {
+            StrLikeLiteral::SingleQuote(_) => Err((
+                progress,
+                EString::ExpectedDoubleQuoteGotSingleQuote(str_like.region.start()),
+            )),
+            StrLikeLiteral::Str(str_literal) => Ok((progress, str_literal, state)),
+        },
+    )
+}
+
+pub fn parse_str_like_literal<'a>() -> impl Parser<'a, StrLikeLiteral<'a>, EString<'a>> {
     move |arena: &'a Bump, mut state: State<'a>, min_indent: u32| {
         let is_multiline;
+        let is_single_quote;
 
         let indent = state.column();
 
@@ -171,6 +98,7 @@ pub fn parse<'a>() -> impl Parser<'a, StrLiteral<'a>, EString<'a>> {
 
             // we will be parsing a multi-line string
             is_multiline = true;
+            is_single_quote = false;
 
             if state.consume_mut("\n") {
                 state = consume_indent(state, indent)?;
@@ -180,6 +108,12 @@ pub fn parse<'a>() -> impl Parser<'a, StrLiteral<'a>, EString<'a>> {
 
             // we will be parsing a single-line string
             is_multiline = false;
+            is_single_quote = false;
+        } else if state.consume_mut("'") {
+            start_state = state.clone();
+
+            is_multiline = false;
+            is_single_quote = true;
         } else {
             return Err((NoProgress, EString::Open(state.pos())));
         }
@@ -244,12 +178,16 @@ pub fn parse<'a>() -> impl Parser<'a, StrLiteral<'a>, EString<'a>> {
             segment_parsed_bytes += 1;
 
             match byte {
-                b'"' => {
+                b'"' if !is_single_quote => {
                     if segment_parsed_bytes == 1 && segments.is_empty() {
                         // special case of the empty string
                         if is_multiline {
                             if bytes.as_slice().starts_with(b"\"\"") {
-                                return Ok((MadeProgress, Block(&[]), state.advance(3)));
+                                return Ok((
+                                    MadeProgress,
+                                    StrLikeLiteral::Str(StrLiteral::Block(&[])),
+                                    state.advance(3),
+                                ));
                             } else {
                                 // this quote is in a block string
                                 continue;
@@ -257,7 +195,11 @@ pub fn parse<'a>() -> impl Parser<'a, StrLiteral<'a>, EString<'a>> {
                         } else {
                             // This is the end of the string!
                             // Advance 1 for the close quote
-                            return Ok((MadeProgress, PlainLine(""), state.advance(1)));
+                            return Ok((
+                                MadeProgress,
+                                StrLikeLiteral::Str(StrLiteral::PlainLine("")),
+                                state.advance(1),
+                            ));
                         }
                     } else {
                         // the string is non-empty, which means we need to convert any previous segments
@@ -276,10 +218,14 @@ pub fn parse<'a>() -> impl Parser<'a, StrLiteral<'a>, EString<'a>> {
                                         other => StrLiteral::Line(arena.alloc([other])),
                                     }
                                 } else {
-                                    Block(arena.alloc([segments.into_bump_slice()]))
+                                    StrLiteral::Block(arena.alloc([segments.into_bump_slice()]))
                                 };
 
-                                return Ok((MadeProgress, expr, state.advance(3)));
+                                return Ok((
+                                    MadeProgress,
+                                    StrLikeLiteral::Str(expr),
+                                    state.advance(3),
+                                ));
                             } else {
                                 // this quote is in a block string
                                 continue;
@@ -295,14 +241,80 @@ pub fn parse<'a>() -> impl Parser<'a, StrLiteral<'a>, EString<'a>> {
                                     other => StrLiteral::Line(arena.alloc([other])),
                                 }
                             } else {
-                                Line(segments.into_bump_slice())
+                                StrLiteral::Line(segments.into_bump_slice())
                             };
 
                             // Advance the state 1 to account for the closing `"`
-                            return Ok((MadeProgress, expr, state.advance(1)));
+                            return Ok((MadeProgress, StrLikeLiteral::Str(expr), state.advance(1)));
                         }
                     };
                 }
+                b'\'' if is_single_quote => {
+                    end_segment!(StrSegment::Plaintext);
+
+                    let expr = if segments.len() == 1 {
+                        // We had exactly one segment, so this is a candidate
+                        // to be SingleQuoteLiteral::Plaintext
+                        match segments.pop().unwrap() {
+                            StrSegment::Plaintext(string) => SingleQuoteLiteral::PlainLine(string),
+                            other => {
+                                let o = other.try_into().map_err(|e| {
+                                    (
+                                        MadeProgress,
+                                        EString::InvalidSingleQuote(e, start_state.pos()),
+                                    )
+                                })?;
+                                SingleQuoteLiteral::Line(arena.alloc([o]))
+                            }
+                        }
+                    } else {
+                        let mut new_segments = Vec::with_capacity_in(segments.len(), arena);
+                        for segment in segments {
+                            let segment = segment.try_into().map_err(|e| {
+                                (
+                                    MadeProgress,
+                                    EString::InvalidSingleQuote(e, start_state.pos()),
+                                )
+                            })?;
+                            new_segments.push(segment);
+                        }
+
+                        SingleQuoteLiteral::Line(new_segments.into_bump_slice())
+                    };
+
+                    // Validate that the string is a valid char literal.
+                    // Note that currently, we accept anything that:
+                    // * Is between 1 and 5 bytes long
+                    //   -> utf-8 encoding is trivial to extend to 5 bytes, even tho 4 is the technical max
+                    //   -> TODO: do we want to change this?
+                    // * Decodes as valid UTF-8
+                    //   -> Might be a single code point, or multiple code points
+                    //   -> TODO: do we want to change this?
+
+                    // Simply by decoding this, it's guaranteed to be valid utf-8
+                    let text = expr.to_str_in(arena);
+
+                    if text.len() > 5 {
+                        return Err((
+                            MadeProgress,
+                            EString::InvalidSingleQuote(ESingleQuote::TooLong, start_state.pos()),
+                        ));
+                    }
+
+                    if text.is_empty() {
+                        return Err((
+                            MadeProgress,
+                            EString::InvalidSingleQuote(ESingleQuote::Empty, start_state.pos()),
+                        ));
+                    }
+
+                    // Advance the state 1 to account for the closing `'`
+                    return Ok((
+                        MadeProgress,
+                        StrLikeLiteral::SingleQuote(expr),
+                        state.advance(1),
+                    ));
+                }
                 b'\n' => {
                     if is_multiline {
                         let without_newline = &state.bytes()[0..(segment_parsed_bytes - 1)];
@@ -330,7 +342,7 @@ pub fn parse<'a>() -> impl Parser<'a, StrLiteral<'a>, EString<'a>> {
                         // all remaining chars. This will mask all other errors, but
                         // it should make it easiest to debug; the file will be a giant
                         // error starting from where the open quote appeared.
-                        return Err((MadeProgress, EString::EndlessSingle(start_state.pos())));
+                        return Err((MadeProgress, EString::EndlessSingleLine(start_state.pos())));
                     }
                 }
                 b'\\' => {
@@ -407,7 +419,10 @@ pub fn parse<'a>() -> impl Parser<'a, StrLiteral<'a>, EString<'a>> {
                             escaped_char!(EscapedChar::Backslash);
                         }
                         Some(b'"') => {
-                            escaped_char!(EscapedChar::Quote);
+                            escaped_char!(EscapedChar::DoubleQuote);
+                        }
+                        Some(b'\'') => {
+                            escaped_char!(EscapedChar::SingleQuote);
                         }
                         Some(b'r') => {
                             escaped_char!(EscapedChar::CarriageReturn);
@@ -435,10 +450,12 @@ pub fn parse<'a>() -> impl Parser<'a, StrLiteral<'a>, EString<'a>> {
         // We ran out of characters before finding a closed quote
         Err((
             MadeProgress,
-            if is_multiline {
-                EString::EndlessMulti(start_state.pos())
+            if is_single_quote {
+                EString::EndlessSingleQuote(start_state.pos())
+            } else if is_multiline {
+                EString::EndlessMultiLine(start_state.pos())
             } else {
-                EString::EndlessSingle(start_state.pos())
+                EString::EndlessSingleLine(start_state.pos())
             },
         ))
     }

crates/compiler/parse/tests/test_parse.rs

@@ -66,7 +66,7 @@ mod test_parse {
             ("\\n", EscapedChar::Newline),
             ("\\r", EscapedChar::CarriageReturn),
             ("\\t", EscapedChar::Tab),
-            ("\\\"", EscapedChar::Quote),
+            ("\\\"", EscapedChar::DoubleQuote),
         ] {
             let actual = parse_expr_with(&arena, arena.alloc(to_input(string)));
             let expected_slice = to_expected(*escaped, &arena);

crates/compiler/test_syntax/fuzz/Cargo.lock

@@ -448,7 +448,6 @@ dependencies = [
  "roc_module",
  "roc_parse",
  "roc_region",
- "roc_test_utils",
 ]
 
 [[package]]

crates/compiler/test_syntax/tests/snapshots/fail/multi_no_end.expr.result-ast

@@ -1 +1 @@
-Expr(Str(EndlessMulti(@3), @0), @0)
+Expr(Str(EndlessMultiLine(@3), @0), @0)

crates/compiler/test_syntax/tests/snapshots/fail/single_no_end.expr.result-ast

@@ -1 +1 @@
-Expr(Str(EndlessSingle(@1), @0), @0)
+Expr(Str(EndlessSingleLine(@1), @0), @0)

crates/compiler/test_syntax/tests/snapshots/pass/control_characters_in_scalar.expr.formatted.roc

@@ -0,0 +1 @@
+'\u(7)'

crates/compiler/test_syntax/tests/snapshots/pass/control_characters_in_scalar.expr.result-ast

@@ -0,0 +1,3 @@
+SingleQuote(
+    "\u{7}",
+)

crates/compiler/test_syntax/tests/snapshots/pass/control_characters_in_scalar.expr.roc

@@ -0,0 +1 @@
+''

crates/compiler/test_syntax/tests/test_snapshots.rs

@@ -258,6 +258,7 @@ mod test_snapshots {
         pass/comment_before_op.expr,
         pass/comment_inside_empty_list.expr,
         pass/comment_with_non_ascii.expr,
+        pass/control_characters_in_scalar.expr,
         pass/crash.expr,
         pass/dbg.expr,
         pass/def_without_newline.expr,
@@ -567,7 +568,7 @@ mod test_snapshots {
             ("\\n", EscapedChar::Newline),
             ("\\r", EscapedChar::CarriageReturn),
             ("\\t", EscapedChar::Tab),
-            ("\\\"", EscapedChar::Quote),
+            ("\\\"", EscapedChar::DoubleQuote),
         ] {
             let actual = parse_expr_with(&arena, arena.alloc(to_input(string)));
             let expected_slice = to_expected(*escaped, &arena);

crates/reporting/src/error/parse.rs

@@ -1,4 +1,4 @@
-use roc_parse::parser::{ENumber, FileError, PList, SyntaxError};
+use roc_parse::parser::{ENumber, ESingleQuote, FileError, PList, SyntaxError};
 use roc_problem::Severity;
 use roc_region::all::{LineColumn, LineColumnRegion, LineInfo, Position, Region};
 use std::path::PathBuf;
@@ -967,7 +967,96 @@ fn to_str_report<'a>(
                 severity: Severity::RuntimeError,
             }
         }
-        EString::EndlessSingle(pos) => {
+        EString::EndlessSingleQuote(pos) => {
+            let surroundings = Region::new(start, pos);
+            let region = LineColumnRegion::from_pos(lines.convert_pos(pos));
+
+            let doc = alloc.stack([
+                alloc.reflow(r"I cannot find the end of this scalar literal (character literal):"),
+                alloc.region_with_subregion(lines.convert_region(surroundings), region),
+                alloc.concat([
+                    alloc.reflow(r"You could change it to something like "),
+                    alloc.parser_suggestion("'a'"),
+                    alloc.reflow(" or "),
+                    alloc.parser_suggestion("'\n'"),
+                    alloc.reflow("."),
+                ]),
+            ]);
+
+            Report {
+                filename,
+                doc,
+                title: "ENDLESS SCALAR".to_string(),
+                severity: Severity::RuntimeError,
+            }
+        }
+        EString::InvalidSingleQuote(e, pos) => {
+            let surroundings = Region::new(start, pos);
+            let region = LineColumnRegion::from_pos(lines.convert_pos(pos));
+
+            let doc = match e {
+                ESingleQuote::Empty => {
+                    alloc.stack([
+                        alloc.concat([
+                            alloc.reflow(r"I am part way through parsing this scalar literal (character literal), "),
+                            alloc.reflow(r"but it appears to be empty - which is not a valid scalar."),
+                        ]),
+                        alloc.region_with_subregion(lines.convert_region(surroundings), region),
+                        alloc.concat([
+                            alloc.reflow(r"You could change it to something like "),
+                            alloc.parser_suggestion("'a'"),
+                            alloc.reflow(" or "),
+                            alloc.parser_suggestion("'\\n'"),
+                            alloc.reflow(". "),
+                            alloc.reflow("Note, roc strings use double quotes, like \"hello\".")
+                        ]),
+                    ])
+                }
+                ESingleQuote::TooLong => {
+                    alloc.stack([
+                        alloc.concat([
+                            alloc.reflow(r"I am part way through parsing this scalar literal (character literal), "),
+                            alloc.reflow(r"but it's too long to fit in a U32 so it's not a valid scalar."),
+                        ]),
+                        alloc.region_with_subregion(lines.convert_region(surroundings), region),
+                        alloc.concat([
+                            alloc.reflow(r"You could change it to something like "),
+                            alloc.parser_suggestion("'a'"),
+                            alloc.reflow(" or "),
+                            alloc.parser_suggestion("'\\n'"),
+                            alloc.reflow(". "),
+                            alloc.reflow("Note, roc strings use double quotes, like \"hello\".")
+                        ]),
+                    ])
+                }
+                ESingleQuote::InterpolationNotAllowed => {
+                    alloc.stack([
+                        alloc.concat([
+                            alloc.reflow("I am part way through parsing this scalar literal (character literal), "),
+                            alloc.reflow("but I encountered a string interpolation like \"\\(this)\", which is not "),
+                            alloc.reflow("allowed in scalar literals."),
+                        ]),
+                        alloc.region_with_subregion(lines.convert_region(surroundings), region),
+                        alloc.concat([
+                            alloc.reflow(r"You could change it to something like "),
+                            alloc.parser_suggestion("'a'"),
+                            alloc.reflow(" or "),
+                            alloc.parser_suggestion("'\\n'"),
+                            alloc.reflow(". "),
+                            alloc.reflow("Note, roc strings use double quotes, like \"hello\".")
+                        ]),
+                    ])
+                }
+            };
+
+            Report {
+                filename,
+                doc,
+                title: "INVALID SCALAR".to_string(),
+                severity: Severity::RuntimeError,
+            }
+        }
+        EString::EndlessSingleLine(pos) => {
             let surroundings = Region::new(start, pos);
             let region = LineColumnRegion::from_pos(lines.convert_pos(pos));
 
@@ -990,7 +1079,31 @@ fn to_str_report<'a>(
                 severity: Severity::RuntimeError,
             }
         }
-        EString::EndlessMulti(pos) => {
+        EString::ExpectedDoubleQuoteGotSingleQuote(pos) => {
+            let surroundings = Region::new(start, pos);
+            let region = LineColumnRegion::from_pos(lines.convert_pos(pos));
+
+            let doc = alloc.stack([
+                alloc.reflow(r"I was expecting to see a string here, but I got a scalar literal."),
+                alloc.region_with_subregion(lines.convert_region(surroundings), region),
+                alloc.concat([
+                    alloc.reflow(r"You could change it to something like "),
+                    alloc.parser_suggestion("\"to be or not to be\""),
+                    alloc.reflow(" or even just "),
+                    alloc.parser_suggestion("\"\""),
+                    alloc.reflow(". "),
+                    alloc.reflow("Note, roc strings use double quotes."),
+                ]),
+            ]);
+
+            Report {
+                filename,
+                doc,
+                title: "EXPECTED STRING".to_string(),
+                severity: Severity::RuntimeError,
+            }
+        }
+        EString::EndlessMultiLine(pos) => {
             let surroundings = Region::new(start, pos);
             let region = LineColumnRegion::from_pos(lines.convert_pos(pos));
 

crates/reporting/tests/test_reporting.rs

@@ -5303,6 +5303,23 @@ Tab characters are not allowed."###,
     "###
     );
 
+    test_report!(
+        single_quote_too_long,
+        r#"'abcdef'"#,
+        @r###"
+    ── INVALID SCALAR ───────────────────────── tmp/single_quote_too_long/Test.roc ─
+
+    I am part way through parsing this scalar literal (character literal),
+    but it's too long to fit in a U32 so it's not a valid scalar.
+
+    4│      'abcdef'
+             ^
+
+    You could change it to something like 'a' or '\n'. Note, roc strings
+    use double quotes, like "hello".
+    "###
+    );
+
     test_report!(
         single_no_end,
         r#""there is no end"#,