Add support for turning off constructs

24 files changed, 353 insertions, 62 deletions

diff --git a/src/construct/attention.rs b/src/construct/attention.rs
index 86c9249..3e15f9a 100644
--- a/src/construct/attention.rs
+++ b/src/construct/attention.rs
@@ -175,7 +175,7 @@ struct Sequence {
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
-        Code::Char('*' | '_') => {
+        Code::Char('*' | '_') if tokenizer.parse_state.constructs.attention => {
             tokenizer.enter(Token::AttentionSequence);
             inside(tokenizer, code, MarkerKind::from_code(code))
         }
diff --git a/src/construct/autolink.rs b/src/construct/autolink.rs
index fe8f380..0ef4607 100644
--- a/src/construct/autolink.rs
+++ b/src/construct/autolink.rs
@@ -115,7 +115,7 @@ use crate::tokenizer::{Code, State, StateFnResult, Tokenizer};
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
-        Code::Char('<') => {
+        Code::Char('<') if tokenizer.parse_state.constructs.autolink => {
             tokenizer.enter(Token::Autolink);
             tokenizer.enter(Token::AutolinkMarker);
             tokenizer.consume(code);
diff --git a/src/construct/block_quote.rs b/src/construct/block_quote.rs
index 6c3f270..a8a8fa8 100644
--- a/src/construct/block_quote.rs
+++ b/src/construct/block_quote.rs
@@ -45,8 +45,16 @@ use crate::tokenizer::{Code, State, StateFnResult, Tokenizer};
 ///     ^
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    // To do: allow arbitrary when code (indented) is turned off.
-    tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), before)(tokenizer, code)
+    let max = if tokenizer.parse_state.constructs.code_indented {
+        TAB_SIZE - 1
+    } else {
+        usize::MAX
+    };
+    if tokenizer.parse_state.constructs.block_quote {
+        tokenizer.go(space_or_tab_min_max(0, max), before)(tokenizer, code)
+    } else {
+        (State::Nok, None)
+    }
 }
 
 /// Start of block quote, after whitespace, before `>`.
@@ -73,8 +81,12 @@ fn before(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
 ///     ^
 /// ```
 pub fn cont(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    // To do: allow arbitrary when code (indented) is turned off.
-    tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), cont_before)(tokenizer, code)
+    let max = if tokenizer.parse_state.constructs.code_indented {
+        TAB_SIZE - 1
+    } else {
+        usize::MAX
+    };
+    tokenizer.go(space_or_tab_min_max(0, max), cont_before)(tokenizer, code)
 }
 
 /// After whitespace, before `>`.
diff --git a/src/construct/character_escape.rs b/src/construct/character_escape.rs
index 811adcf..f171d38 100644
--- a/src/construct/character_escape.rs
+++ b/src/construct/character_escape.rs
@@ -44,7 +44,7 @@ use crate::tokenizer::{Code, State, StateFnResult, Tokenizer};
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
-        Code::Char('\\') => {
+        Code::Char('\\') if tokenizer.parse_state.constructs.character_escape => {
             tokenizer.enter(Token::CharacterEscape);
             tokenizer.enter(Token::CharacterEscapeMarker);
             tokenizer.consume(code);
diff --git a/src/construct/character_reference.rs b/src/construct/character_reference.rs
index 544458a..ce7cd31 100644
--- a/src/construct/character_reference.rs
+++ b/src/construct/character_reference.rs
@@ -138,7 +138,7 @@ struct Info {
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
-        Code::Char('&') => {
+        Code::Char('&') if tokenizer.parse_state.constructs.character_reference => {
             tokenizer.enter(Token::CharacterReference);
             tokenizer.enter(Token::CharacterReferenceMarker);
             tokenizer.consume(code);
diff --git a/src/construct/code_fenced.rs b/src/construct/code_fenced.rs
index d5001e7..49bcae3 100644
--- a/src/construct/code_fenced.rs
+++ b/src/construct/code_fenced.rs
@@ -189,10 +189,18 @@ struct Info {
 ///   | ~~~
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    tokenizer.enter(Token::CodeFenced);
-    tokenizer.enter(Token::CodeFencedFence);
-    // To do: allow arbitrary when code (indented) is turned off.
-    tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), before_sequence_open)(tokenizer, code)
+    let max = if tokenizer.parse_state.constructs.code_indented {
+        TAB_SIZE - 1
+    } else {
+        usize::MAX
+    };
+    if tokenizer.parse_state.constructs.code_fenced {
+        tokenizer.enter(Token::CodeFenced);
+        tokenizer.enter(Token::CodeFencedFence);
+        tokenizer.go(space_or_tab_min_max(0, max), before_sequence_open)(tokenizer, code)
+    } else {
+        (State::Nok, None)
+    }
 }
 
 /// Inside the opening fence, after an optional prefix, before a sequence.
@@ -445,10 +453,20 @@ fn close_begin(tokenizer: &mut Tokenizer, code: Code, info: Info) -> StateFnResu
 ///     ^
 /// ```
 fn close_start(tokenizer: &mut Tokenizer, code: Code, info: Info) -> StateFnResult {
-    tokenizer.enter(Token::CodeFencedFence);
-    tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), |t, c| {
-        close_before(t, c, info)
-    })(tokenizer, code)
+    let max = if tokenizer.parse_state.constructs.code_indented {
+        TAB_SIZE - 1
+    } else {
+        usize::MAX
+    };
+
+    if tokenizer.parse_state.constructs.code_fenced {
+        tokenizer.enter(Token::CodeFencedFence);
+        tokenizer.go(space_or_tab_min_max(0, max), |t, c| {
+            close_before(t, c, info)
+        })(tokenizer, code)
+    } else {
+        (State::Nok, None)
+    }
 }
 
 /// In a closing fence, after optional whitespace, before sequence.
diff --git a/src/construct/code_indented.rs b/src/construct/code_indented.rs
index 6778b62..eb0811b 100644
--- a/src/construct/code_indented.rs
+++ b/src/construct/code_indented.rs
@@ -62,7 +62,7 @@ use crate::tokenizer::{Code, State, StateFnResult, Tokenizer};
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     // Do not interrupt paragraphs.
-    if tokenizer.interrupt {
+    if tokenizer.interrupt || !tokenizer.parse_state.constructs.code_indented {
         (State::Nok, None)
     } else {
         tokenizer.enter(Token::CodeIndented);
diff --git a/src/construct/code_text.rs b/src/construct/code_text.rs
index 5e40d03..eb143ba 100644
--- a/src/construct/code_text.rs
+++ b/src/construct/code_text.rs
@@ -99,8 +99,10 @@ pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
 
     match code {
         Code::Char('`')
-            if tokenizer.previous != Code::Char('`')
-                || (len > 0 && tokenizer.events[len - 1].token_type == Token::CharacterEscape) =>
+            if tokenizer.parse_state.constructs.code_text
+                && (tokenizer.previous != Code::Char('`')
+                    || (len > 0
+                        && tokenizer.events[len - 1].token_type == Token::CharacterEscape)) =>
         {
             tokenizer.enter(Token::CodeText);
             tokenizer.enter(Token::CodeTextSequence);
diff --git a/src/construct/definition.rs b/src/construct/definition.rs
index 6ce3a04..231011f 100644
--- a/src/construct/definition.rs
+++ b/src/construct/definition.rs
@@ -120,12 +120,12 @@ pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
             == Token::Definition;
 
     // Do not interrupt paragraphs (but do follow definitions).
-    if tokenizer.interrupt && !definition_before {
-        (State::Nok, None)
-    } else {
+    if (!tokenizer.interrupt || definition_before) && tokenizer.parse_state.constructs.definition {
         tokenizer.enter(Token::Definition);
         // Note: arbitrary whitespace allowed even if code (indented) is on.
         tokenizer.attempt_opt(space_or_tab(), before)(tokenizer, code)
+    } else {
+        (State::Nok, None)
     }
 }
 
diff --git a/src/construct/hard_break_escape.rs b/src/construct/hard_break_escape.rs
index de8afe6..191ef67 100644
--- a/src/construct/hard_break_escape.rs
+++ b/src/construct/hard_break_escape.rs
@@ -52,7 +52,7 @@ use crate::tokenizer::{Code, State, StateFnResult, Tokenizer};
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
-        Code::Char('\\') => {
+        Code::Char('\\') if tokenizer.parse_state.constructs.hard_break_escape => {
             tokenizer.enter(Token::HardBreakEscape);
             tokenizer.enter(Token::HardBreakEscapeMarker);
             tokenizer.consume(code);
diff --git a/src/construct/hard_break_trailing.rs b/src/construct/hard_break_trailing.rs
index d83bf60..88c668a 100644
--- a/src/construct/hard_break_trailing.rs
+++ b/src/construct/hard_break_trailing.rs
@@ -53,7 +53,7 @@ use crate::tokenizer::{Code, State, StateFnResult, Tokenizer};
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
-        Code::Char(' ') => {
+        Code::Char(' ') if tokenizer.parse_state.constructs.hard_break_trailing => {
             tokenizer.enter(Token::HardBreakTrailing);
             tokenizer.enter(Token::HardBreakTrailingSpace);
             tokenizer.consume(code);
diff --git a/src/construct/heading_atx.rs b/src/construct/heading_atx.rs
index 8947f64..68a6be7 100644
--- a/src/construct/heading_atx.rs
+++ b/src/construct/heading_atx.rs
@@ -67,9 +67,18 @@ use crate::util::edit_map::EditMap;
 ///     ^
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    tokenizer.enter(Token::HeadingAtx);
-    // To do: allow arbitrary when code (indented) is turned off.
-    tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), before)(tokenizer, code)
+    let max = if tokenizer.parse_state.constructs.code_indented {
+        TAB_SIZE - 1
+    } else {
+        usize::MAX
+    };
+
+    if tokenizer.parse_state.constructs.heading_atx {
+        tokenizer.enter(Token::HeadingAtx);
+        tokenizer.go(space_or_tab_min_max(0, max), before)(tokenizer, code)
+    } else {
+        (State::Nok, None)
+    }
 }
 
 /// Start of a heading (atx), after whitespace.
diff --git a/src/construct/heading_setext.rs b/src/construct/heading_setext.rs
index deab558..7cd259b 100644
--- a/src/construct/heading_setext.rs
+++ b/src/construct/heading_setext.rs
@@ -117,6 +117,11 @@ impl Kind {
 ///     ^
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    let max = if tokenizer.parse_state.constructs.code_indented {
+        TAB_SIZE - 1
+    } else {
+        usize::MAX
+    };
     let paragraph_before = !tokenizer.events.is_empty()
         && tokenizer.events[skip_opt_back(
             &tokenizer.events,
@@ -127,9 +132,8 @@ pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
             == Token::Paragraph;
 
     // Require a paragraph before and do not allow on a lazy line.
-    if paragraph_before && !tokenizer.lazy {
-        // To do: allow arbitrary when code (indented) is turned off.
-        tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), before)(tokenizer, code)
+    if paragraph_before && !tokenizer.lazy && tokenizer.parse_state.constructs.heading_setext {
+        tokenizer.go(space_or_tab_min_max(0, max), before)(tokenizer, code)
     } else {
         (State::Nok, None)
     }
diff --git a/src/construct/html_flow.rs b/src/construct/html_flow.rs
index 822b9dd..1255081 100644
--- a/src/construct/html_flow.rs
+++ b/src/construct/html_flow.rs
@@ -204,18 +204,27 @@ struct Info {
 ///     ^
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    tokenizer.enter(Token::HtmlFlow);
-    // To do: allow arbitrary when code (indented) is turned off.
-    tokenizer.go(
-        space_or_tab_with_options(SpaceOrTabOptions {
-            kind: Token::HtmlFlowData,
-            min: 0,
-            max: TAB_SIZE - 1,
-            connect: false,
-            content_type: None,
-        }),
-        before,
-    )(tokenizer, code)
+    let max = if tokenizer.parse_state.constructs.code_indented {
+        TAB_SIZE - 1
+    } else {
+        usize::MAX
+    };
+
+    if tokenizer.parse_state.constructs.html_flow {
+        tokenizer.enter(Token::HtmlFlow);
+        tokenizer.go(
+            space_or_tab_with_options(SpaceOrTabOptions {
+                kind: Token::HtmlFlowData,
+                min: 0,
+                max,
+                connect: false,
+                content_type: None,
+            }),
+            before,
+        )(tokenizer, code)
+    } else {
+        (State::Nok, None)
+    }
 }
 
 /// After optional whitespace, before `<`.
diff --git a/src/construct/html_text.rs b/src/construct/html_text.rs
index be1f1fe..db00551 100644
--- a/src/construct/html_text.rs
+++ b/src/construct/html_text.rs
@@ -66,7 +66,7 @@ use crate::util::codes::parse;
 ///       ^
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    if Code::Char('<') == code {
+    if Code::Char('<') == code && tokenizer.parse_state.constructs.html_text {
         tokenizer.enter(Token::HtmlText);
         tokenizer.enter(Token::HtmlTextData);
         tokenizer.consume(code);
diff --git a/src/construct/label_end.rs b/src/construct/label_end.rs
index e232cbe..2ac2500 100644
--- a/src/construct/label_end.rs
+++ b/src/construct/label_end.rs
@@ -184,7 +184,7 @@ struct Info {
 /// > | [a] b
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    if Code::Char(']') == code {
+    if Code::Char(']') == code && tokenizer.parse_state.constructs.label_end {
         let mut label_start_index: Option<usize> = None;
         let mut index = tokenizer.label_start_stack.len();
 
diff --git a/src/construct/label_start_image.rs b/src/construct/label_start_image.rs
index b4e0433..fd7a42d 100644
--- a/src/construct/label_start_image.rs
+++ b/src/construct/label_start_image.rs
@@ -40,7 +40,7 @@ use crate::tokenizer::{Code, LabelStart, State, StateFnResult, Tokenizer};
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
-        Code::Char('!') => {
+        Code::Char('!') if tokenizer.parse_state.constructs.label_start_image => {
             tokenizer.enter(Token::LabelImage);
             tokenizer.enter(Token::LabelImageMarker);
             tokenizer.consume(code);
diff --git a/src/construct/label_start_link.rs b/src/construct/label_start_link.rs
index 7e8e511..aeaa4eb 100644
--- a/src/construct/label_start_link.rs
+++ b/src/construct/label_start_link.rs
@@ -39,7 +39,7 @@ use crate::tokenizer::{Code, LabelStart, State, StateFnResult, Tokenizer};
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
-        Code::Char('[') => {
+        Code::Char('[') if tokenizer.parse_state.constructs.label_start_link => {
             let start = tokenizer.events.len();
             tokenizer.enter(Token::LabelLink);
             tokenizer.enter(Token::LabelMarker);
diff --git a/src/construct/list.rs b/src/construct/list.rs
index fce8f00..5fd0849 100644
--- a/src/construct/list.rs
+++ b/src/construct/list.rs
@@ -138,9 +138,18 @@ impl Kind {
 ///     ^
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    tokenizer.enter(Token::ListItem);
-    // To do: allow arbitrary when code (indented) is turned off.
-    tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), before)(tokenizer, code)
+    let max = if tokenizer.parse_state.constructs.code_indented {
+        TAB_SIZE - 1
+    } else {
+        usize::MAX
+    };
+
+    if tokenizer.parse_state.constructs.list {
+        tokenizer.enter(Token::ListItem);
+        tokenizer.go(space_or_tab_min_max(0, max), before)(tokenizer, code)
+    } else {
+        (State::Nok, None)
+    }
 }
 
 /// Start of list item, after whitespace.
diff --git a/src/construct/thematic_break.rs b/src/construct/thematic_break.rs
index f0b6052..48fb838 100644
--- a/src/construct/thematic_break.rs
+++ b/src/construct/thematic_break.rs
@@ -135,9 +135,18 @@ struct Info {
 ///     ^
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    tokenizer.enter(Token::ThematicBreak);
-    // To do: allow arbitrary when code (indented) is turned off.
-    tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), before)(tokenizer, code)
+    let max = if tokenizer.parse_state.constructs.code_indented {
+        TAB_SIZE - 1
+    } else {
+        usize::MAX
+    };
+
+    if tokenizer.parse_state.constructs.thematic_break {
+        tokenizer.enter(Token::ThematicBreak);
+        tokenizer.go(space_or_tab_min_max(0, max), before)(tokenizer, code)
+    } else {
+        (State::Nok, None)
+    }
 }
 
 /// Start of a thematic break, after whitespace.
diff --git a/src/content/string.rs b/src/content/string.rs
index cc8ee53..f2650df 100644
--- a/src/content/string.rs
+++ b/src/content/string.rs
@@ -19,6 +19,7 @@ use crate::construct::{
 use crate::tokenizer::{Code, State, StateFnResult, Tokenizer};
 
 const MARKERS: [Code; 5] = [
+    // To do: build this vec based on whether they are enabled?
     Code::VirtualSpace, // `whitespace`
     Code::Char('\t'),   // `whitespace`
     Code::Char(' '),    // `whitespace`
diff --git a/src/content/text.rs b/src/content/text.rs
index cf630f1..f797b11 100644
--- a/src/content/text.rs
+++ b/src/content/text.rs
@@ -58,6 +58,7 @@ pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
         Code::None => (State::Ok, None),
         _ => tokenizer.attempt_n(
             vec![
+                // To do: build this vec based on whether they are enabled?
                 Box::new(attention),
                 Box::new(autolink),
                 Box::new(character_escape),
diff --git a/src/lib.rs b/src/lib.rs
index ff8e938..224f8d0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -75,8 +75,193 @@ impl LineEnding {
     }
 }
 
+/// Control which constructs are enabled.
+///
+/// Not all constructs can be configured.
+/// Notably, blank lines and paragraphs cannot be turned off.
+#[allow(clippy::struct_excessive_bools)]
+#[derive(Clone, Debug)]
+pub struct Constructs {
+    /// Attention.
+    ///
+    /// ```markdown
+    /// > | a *b* c **d**.
+    ///       ^^^   ^^^^^
+    /// ```
+    pub attention: bool,
+    /// Autolink.
+    ///
+    /// ```markdown
+    /// > | a <https://example.com> b <user@example.org>.
+    ///       ^^^^^^^^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^^^^^
+    /// ```
+    pub autolink: bool,
+    /// Block quote.
+    ///
+    /// ```markdown
+    /// > | > a
+    ///     ^^^
+    /// ```
+    pub block_quote: bool,
+    /// Character escape.
+    ///
+    /// ```markdown
+    /// > | a \* b
+    ///       ^^
+    /// ```
+    pub character_escape: bool,
+    /// Character reference.
+    ///
+    /// ```markdown
+    /// > | a &amp; b
+    ///       ^^^^^
+    /// ```
+    pub character_reference: bool,
+    /// Code (indented).
+    ///
+    /// ```markdown
+    /// > |     a
+    ///     ^^^^^
+    /// ```
+    pub code_indented: bool,
+    /// Code (fenced).
+    ///
+    /// ```markdown
+    /// > | ~~~js
+    ///     ^^^^^
+    /// > | console.log(1)
+    ///     ^^^^^^^^^^^^^^
+    /// > | ~~~
+    ///     ^^^
+    /// ```
+    pub code_fenced: bool,
+    /// Code (text).
+    ///
+    /// ```markdown
+    /// > | a `b` c
+    ///       ^^^
+    /// ```
+    pub code_text: bool,
+    /// Definition.
+    ///
+    /// ```markdown
+    /// > | [a]: b "c"
+    ///     ^^^^^^^^^^
+    /// ```
+    pub definition: bool,
+    /// Hard break (escape).
+    ///
+    /// ```markdown
+    /// > | a\
+    ///      ^
+    ///   | b
+    /// ```
+    pub hard_break_escape: bool,
+    /// Hard break (trailing).
+    ///
+    /// ```markdown
+    /// > | a␠␠
+    ///      ^^
+    ///   | b
+    /// ```
+    pub hard_break_trailing: bool,
+    /// Heading (atx).
+    ///
+    /// ```markdown
+    /// > | # a
+    ///     ^^^
+    /// ```
+    pub heading_atx: bool,
+    /// Heading (setext).
+    ///
+    /// ```markdown
+    /// > | a
+    ///     ^^
+    /// > | ==
+    ///     ^^
+    /// ```
+    pub heading_setext: bool,
+    /// HTML (flow).
+    ///
+    /// ```markdown
+    /// > | <div>
+    ///     ^^^^^
+    /// ```
+    pub html_flow: bool,
+    /// HTML (text).
+    ///
+    /// ```markdown
+    /// > | a <b> c
+    ///       ^^^
+    /// ```
+    pub html_text: bool,
+    /// Label start (image).
+    ///
+    /// ```markdown
+    /// > | a ![b](c) d
+    ///       ^^
+    /// ```
+    pub label_start_image: bool,
+    /// Label start (link).
+    ///
+    /// ```markdown
+    /// > | a [b](c) d
+    ///       ^
+    /// ```
+    pub label_start_link: bool,
+    /// Label end.
+    ///
+    /// ```markdown
+    /// > | a [b](c) d
+    ///         ^^^^
+    /// ```
+    pub label_end: bool,
+    /// List.
+    ///
+    /// ```markdown
+    /// > | * a
+    ///     ^^^
+    /// ```
+    pub list: bool,
+    /// Thematic break.
+    ///
+    /// ```markdown
+    /// > | ***
+    ///     ^^^
+    /// ```
+    pub thematic_break: bool,
+}
+
+impl Default for Constructs {
+    /// `CommonMark`.
+    fn default() -> Self {
+        Self {
+            attention: true,
+            autolink: true,
+            block_quote: true,
+            character_escape: true,
+            character_reference: true,
+            code_indented: true,
+            code_fenced: true,
+            code_text: true,
+            definition: true,
+            hard_break_escape: true,
+            hard_break_trailing: true,
+            heading_atx: true,
+            heading_setext: true,
+            html_flow: true,
+            html_text: true,
+            label_start_image: true,
+            label_start_link: true,
+            label_end: true,
+            list: true,
+            thematic_break: true,
+        }
+    }
+}
+
 /// Configuration (optional).
-#[derive(Default, Debug)]
+#[derive(Clone, Debug, Default)]
 pub struct Options {
     /// Whether to allow (dangerous) HTML.
     /// The default is `false`, you can turn it on to `true` for trusted
@@ -99,8 +284,7 @@ pub struct Options {
     ///         "Hi, <i>venus</i>!",
     ///         &Options {
     ///             allow_dangerous_html: true,
-    ///             allow_dangerous_protocol: false,
-    ///             default_line_ending: None,
+    ///             ..Options::default()
     ///         }
     ///     ),
     ///     "<p>Hi, <i>venus</i>!</p>"
@@ -128,9 +312,8 @@ pub struct Options {
     ///     micromark_with_options(
     ///         "<javascript:alert(1)>",
     ///         &Options {
-    ///             allow_dangerous_html: false,
     ///             allow_dangerous_protocol: true,
-    ///             default_line_ending: None,
+    ///             ..Options::default()
     ///         }
     ///     ),
     ///     "<p><a href=\"javascript:alert(1)\">javascript:alert(1)</a></p>"
@@ -166,15 +349,46 @@ pub struct Options {
     ///     micromark_with_options(
     ///         "> a",
     ///         &Options {
-    ///             allow_dangerous_html: false,
-    ///             allow_dangerous_protocol: false,
     ///             default_line_ending: Some(LineEnding::CarriageReturnLineFeed),
+    ///             ..Options::default()
     ///         }
     ///     ),
     ///     "<blockquote>\r\n<p>a</p>\r\n</blockquote>"
     /// );
     /// ```
+    // To do: use `default`? <https://doc.rust-lang.org/std/default/trait.Default.html#enums>
     pub default_line_ending: Option<LineEnding>,
+
+    /// Which constructs to enable and disable.
+    /// The default is to follow `CommonMark`.
+    ///
+    /// ## Examples
+    ///
+    /// ```rust
+    /// use micromark::{micromark, micromark_with_options, Options, Constructs};
+    ///
+    /// // micromark follows CommonMark by default:
+    /// assert_eq!(
+    ///     micromark("    indented code?"),
+    ///     "<pre><code>indented code?\n</code></pre>"
+    /// );
+    ///
+    /// // Pass `constructs` to choose what to enable and disable:
+    /// assert_eq!(
+    ///     micromark_with_options(
+    ///         "    indented code?",
+    ///         &Options {
+    ///             constructs: Constructs {
+    ///                 code_indented: false,
+    ///                 ..Constructs::default()
+    ///             },
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p>indented code?</p>"
+    /// );
+    /// ```
+    pub constructs: Constructs,
 }
 
 /// Turn markdown into HTML.
@@ -203,13 +417,13 @@ pub fn micromark(value: &str) -> String {
 /// let result = micromark_with_options("<div>\n\n# Hello, world!\n\n</div>", &Options {
 ///     allow_dangerous_html: true,
 ///     allow_dangerous_protocol: true,
-///     default_line_ending: None,
+///     ..Options::default()
 /// });
 ///
 /// assert_eq!(result, "<div>\n<h1>Hello, world!</h1>\n</div>");
 /// ```
 #[must_use]
 pub fn micromark_with_options(value: &str, options: &Options) -> String {
-    let (events, result) = parse(value);
+    let (events, result) = parse(value, options);
     compile(&events, &result.codes, options)
 }
diff --git a/src/parser.rs b/src/parser.rs
index 725f326..409e812 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -3,6 +3,7 @@
 use crate::content::document::document;
 use crate::tokenizer::{Code, Event, Point};
 use crate::util::codes::parse as parse_codes;
+use crate::{Constructs, Options};
 use std::collections::HashSet;
 
 /// Information needed, in all content types, when parsing markdown.
@@ -11,6 +12,7 @@ use std::collections::HashSet;
 /// It also references the input value as [`Code`][]s.
 #[derive(Debug)]
 pub struct ParseState {
+    pub constructs: Constructs,
     /// List of codes.
     pub codes: Vec<Code>,
     /// Set of defined identifiers.
@@ -20,8 +22,9 @@ pub struct ParseState {
 /// Turn a string of markdown into events.
 ///
 /// Passes the codes back so the compiler can access the source.
-pub fn parse(value: &str) -> (Vec<Event>, ParseState) {
+pub fn parse(value: &str, options: &Options) -> (Vec<Event>, ParseState) {
     let mut parse_state = ParseState {
+        constructs: options.constructs.clone(),
         codes: parse_codes(value),
         definitions: HashSet::new(),
     };