Allow a function as an argument to size in stretch and lr

Previously there was always a short fall when scaling delimiters, even if
the user requested a specific size. This is no longer the case; the short
fall is only present in the default for `lr` (`x => x - 0.1em`), and is
the reason for the updated tests - the size of the delimiters is now
actually what was specified in the size argument. This also makes the
default for `lr` much clearer to the user.

A slight hack was used by exploiting the `name` property in the `func`
attribute macro so that the default value in the docs for `lr.size` would
clearly show what the default function was (instead of just its name
`default_lr_size` which is meaningless and inaccessible to the user).

crates/typst-layout/src/math/attach.rs

@@ -1,8 +1,9 @@
 use typst_library::diag::SourceResult;
 use typst_library::foundations::{Packed, StyleChain, SymbolElem};
-use typst_library::layout::{Abs, Axis, Corner, Frame, Point, Rel, Size};
+use typst_library::layout::{Abs, Axis, Corner, Frame, Point, Size};
 use typst_library::math::{
     AttachElem, EquationElem, LimitsElem, PrimesElem, ScriptsElem, StretchElem,
+    StretchSize,
 };
 use typst_utils::OptionExt;
 
@@ -63,16 +64,9 @@ pub fn layout_attach(
     let t = layout!(t, sup_style_chain)?;
     let b = layout!(b, sub_style_chain)?;
     if let Some(stretch) = stretch {
-        let relative_to_width = measure!(t, width).max(measure!(b, width));
-        stretch_fragment(
-            ctx,
-            styles,
-            &mut base,
-            Some(Axis::X),
-            Some(relative_to_width),
-            stretch,
-            Abs::zero(),
-        );
+        let relative_to = measure!(t, width).max(measure!(b, width));
+        let width = stretch.resolve(ctx.engine, styles, relative_to)?;
+        stretch_fragment(ctx, styles, &mut base, Axis::X, width);
     }
 
     let fragments = [
@@ -155,7 +149,7 @@ pub fn layout_limits(
 }
 
 /// Get the size to stretch the base to.
-fn stretch_size(styles: StyleChain, elem: &Packed<AttachElem>) -> Option<Rel<Abs>> {
+fn stretch_size(styles: StyleChain, elem: &Packed<AttachElem>) -> Option<StretchSize> {
     // Extract from an EquationElem.
     let mut base = &elem.base;
     while let Some(equation) = base.to_packed::<EquationElem>() {

crates/typst-layout/src/math/fragment.rs

@@ -305,7 +305,7 @@ impl GlyphFragment {
     }
 
     /// Apply GSUB substitutions.
-    fn adjust_glyph_index(ctx: &MathContext, id: GlyphId) -> GlyphId {
+    pub fn adjust_glyph_index(ctx: &MathContext, id: GlyphId) -> GlyphId {
         if let Some(glyphwise_tables) = &ctx.glyphwise_tables {
             glyphwise_tables.iter().fold(id, |id, table| table.apply(id))
         } else {

crates/typst-layout/src/math/lr.rs

@@ -1,11 +1,11 @@
 use typst_library::diag::SourceResult;
 use typst_library::foundations::{Packed, StyleChain};
-use typst_library::layout::{Abs, Axis, Rel};
+use typst_library::layout::{Abs, Axis};
 use typst_library::math::{EquationElem, LrElem, MidElem};
 use typst_utils::SliceExt;
 use unicode_math_class::MathClass;
 
-use super::{stretch_fragment, MathContext, MathFragment, DELIM_SHORT_FALL};
+use super::{stretch_fragment, MathContext, MathFragment};
 
 /// Lays out an [`LrElem`].
 #[typst_macros::time(name = "math.lr", span = elem.span())]
@@ -22,7 +22,7 @@ pub fn layout_lr(
 
     // Extract implicit LrElem.
     if let Some(lr) = body.to_packed::<LrElem>() {
-        if lr.size(styles).is_one() {
+        if lr.size(styles).is_lr_default() {
             body = &lr.body;
         }
     }
@@ -41,14 +41,14 @@ pub fn layout_lr(
         .unwrap_or_default();
 
     let relative_to = 2.0 * max_extent;
-    let height = elem.size(styles);
+    let height = elem.size(styles).resolve(ctx.engine, styles, relative_to)?;
 
     // Scale up fragments at both ends.
     match inner_fragments {
-        [one] => scale(ctx, styles, one, relative_to, height, None),
+        [one] => scale(ctx, styles, one, height, None),
         [first, .., last] => {
-            scale(ctx, styles, first, relative_to, height, Some(MathClass::Opening));
-            scale(ctx, styles, last, relative_to, height, Some(MathClass::Closing));
+            scale(ctx, styles, first, height, Some(MathClass::Opening));
+            scale(ctx, styles, last, height, Some(MathClass::Closing));
         }
         _ => {}
     }
@@ -58,7 +58,7 @@ pub fn layout_lr(
         if let MathFragment::Variant(ref mut variant) = fragment {
             if variant.mid_stretched == Some(false) {
                 variant.mid_stretched = Some(true);
-                scale(ctx, styles, fragment, relative_to, height, Some(MathClass::Large));
+                scale(ctx, styles, fragment, height, Some(MathClass::Large));
             }
         }
     }
@@ -119,26 +119,14 @@ fn scale(
     ctx: &mut MathContext,
     styles: StyleChain,
     fragment: &mut MathFragment,
-    relative_to: Abs,
-    height: Rel<Abs>,
+    height: Abs,
     apply: Option<MathClass>,
 ) {
     if matches!(
         fragment.class(),
         MathClass::Opening | MathClass::Closing | MathClass::Fence
     ) {
-        // This unwrap doesn't really matter. If it is None, then the fragment
-        // won't be stretchable anyways.
-        let short_fall = DELIM_SHORT_FALL.at(fragment.font_size().unwrap_or_default());
-        stretch_fragment(
-            ctx,
-            styles,
-            fragment,
-            Some(Axis::Y),
-            Some(relative_to),
-            height,
-            short_fall,
-        );
+        stretch_fragment(ctx, styles, fragment, Axis::Y, height);
 
         if let Some(class) = apply {
             fragment.set_class(class);

crates/typst-layout/src/math/stretch.rs

@@ -2,7 +2,7 @@ use ttf_parser::math::{GlyphAssembly, GlyphConstruction, GlyphPart};
 use ttf_parser::LazyArray16;
 use typst_library::diag::{warning, SourceResult};
 use typst_library::foundations::{Packed, StyleChain};
-use typst_library::layout::{Abs, Axis, Frame, Point, Rel, Size};
+use typst_library::layout::{Abs, Axis, Frame, Point, Size};
 use typst_library::math::StretchElem;
 use typst_utils::Get;
 
@@ -23,15 +23,13 @@ pub fn layout_stretch(
     styles: StyleChain,
 ) -> SourceResult<()> {
     let mut fragment = ctx.layout_into_fragment(&elem.body, styles)?;
-    stretch_fragment(
-        ctx,
-        styles,
-        &mut fragment,
-        None,
-        None,
-        elem.size(styles),
-        Abs::zero(),
-    );
+
+    if let Some(axis) = stretch_axis(ctx, &fragment) {
+        let relative_to = fragment.size().get(axis);
+        let target = elem.size(styles).resolve(ctx.engine, styles, relative_to)?;
+        stretch_fragment(ctx, styles, &mut fragment, axis, target);
+    }
+
     ctx.push(fragment);
     Ok(())
 }
@@ -41,10 +39,8 @@ pub fn stretch_fragment(
     ctx: &mut MathContext,
     styles: StyleChain,
     fragment: &mut MathFragment,
-    axis: Option<Axis>,
-    relative_to: Option<Abs>,
-    stretch: Rel<Abs>,
-    short_fall: Abs,
+    axis: Axis,
+    target: Abs,
 ) {
     let glyph = match fragment {
         MathFragment::Glyph(glyph) => glyph.clone(),
@@ -56,21 +52,12 @@ pub fn stretch_fragment(
 
     // Return if we attempt to stretch along an axis which isn't stretchable,
     // so that the original fragment isn't modified.
-    let Some(stretch_axis) = stretch_axis(ctx, &glyph) else { return };
-    let axis = axis.unwrap_or(stretch_axis);
+    let Some(stretch_axis) = stretch_axis(ctx, fragment) else { return };
     if axis != stretch_axis {
         return;
     }
 
-    let relative_to_size = relative_to.unwrap_or_else(|| fragment.size().get(axis));
-
-    let mut variant = stretch_glyph(
-        ctx,
-        glyph,
-        stretch.relative_to(relative_to_size),
-        short_fall,
-        axis,
-    );
+    let mut variant = stretch_glyph(ctx, glyph, target, Abs::zero(), axis);
 
     if axis == Axis::Y {
         variant.align_on_axis(ctx, delimiter_alignment(variant.c));
@@ -81,17 +68,26 @@ pub fn stretch_fragment(
 
 /// Return whether the glyph is stretchable and if it is, along which axis it
 /// can be stretched.
-fn stretch_axis(ctx: &mut MathContext, base: &GlyphFragment) -> Option<Axis> {
-    let base_id = base.id;
+fn stretch_axis(ctx: &mut MathContext, fragment: &MathFragment) -> Option<Axis> {
+    let (id, span) = match fragment {
+        MathFragment::Glyph(glyph) => (glyph.id, glyph.span),
+        MathFragment::Variant(variant) => {
+            let id = ctx.ttf.glyph_index(variant.c).unwrap_or_default();
+            let id = GlyphFragment::adjust_glyph_index(ctx, id);
+            (id, variant.span)
+        }
+        _ => return None,
+    };
+
     let vertical = ctx
         .table
         .variants
-        .and_then(|variants| variants.vertical_constructions.get(base_id))
+        .and_then(|variants| variants.vertical_constructions.get(id))
         .map(|_| Axis::Y);
     let horizontal = ctx
         .table
         .variants
-        .and_then(|variants| variants.horizontal_constructions.get(base_id))
+        .and_then(|variants| variants.horizontal_constructions.get(id))
         .map(|_| Axis::X);
 
     match (vertical, horizontal) {
@@ -102,7 +98,7 @@ fn stretch_axis(ctx: &mut MathContext, base: &GlyphFragment) -> Option<Axis> {
             // vertical and horizontal constructions. So for the time being, we
             // will assume that a glyph cannot have both.
             ctx.engine.sink.warn(warning!(
-               base.span,
+               span,
                "glyph has both vertical and horizontal constructions";
                hint: "this is probably a font bug";
                hint: "please file an issue at https://github.com/typst/typst/issues"

crates/typst-library/src/math/attach.rs

@@ -1,5 +1,4 @@
 use crate::foundations::{elem, Content, Packed};
-use crate::layout::{Length, Rel};
 use crate::math::{EquationElem, Mathy};
 
 /// A base with optional attachments.
@@ -128,31 +127,3 @@ pub struct LimitsElem {
     #[default(true)]
     pub inline: bool,
 }
-
-/// Stretches a glyph.
-///
-/// This function can also be used to automatically stretch the base of an
-/// attachment, so that it fits the top and bottom attachments.
-///
-/// Note that only some glyphs can be stretched, and which ones can depend on
-/// the math font being used. However, most math fonts are the same in this
-/// regard.
-///
-/// ```example
-/// $ H stretch(=)^"define" U + p V $
-/// $ f : X stretch(->>, size: #150%)_"surjective" Y $
-/// $ x stretch(harpoons.ltrb, size: #3em) y
-///     stretch(\[, size: #150%) z $
-/// ```
-#[elem(Mathy)]
-pub struct StretchElem {
-    /// The glyph to stretch.
-    #[required]
-    pub body: Content,
-
-    /// The size to stretch to, relative to the maximum size of the glyph and
-    /// its attachments.
-    #[resolve]
-    #[default(Rel::one())]
-    pub size: Rel<Length>,
-}

crates/typst-library/src/math/lr.rs

@@ -1,6 +1,16 @@
-use crate::foundations::{elem, func, Content, NativeElement, SymbolElem};
-use crate::layout::{Length, Rel};
-use crate::math::Mathy;
+use crate::foundations::{elem, func, Content, NativeElement, NativeFunc, SymbolElem};
+use crate::layout::{Em, Length, Ratio, Rel};
+use crate::math::{Mathy, StretchSize};
+
+const DELIM_SHORT_FALL: Em = Em::new(-0.1);
+
+#[func(name = "x => x - 0.1em")]
+pub const fn default_lr_size(base: Length) -> Rel {
+    Rel {
+        rel: Ratio::zero(),
+        abs: Length { abs: base.abs, em: DELIM_SHORT_FALL },
+    }
+}
 
 /// Scales delimiters.
 ///
@@ -8,10 +18,13 @@ use crate::math::Mathy;
 /// unmatched delimiters and to control the delimiter scaling more precisely.
 #[elem(title = "Left/Right", Mathy)]
 pub struct LrElem {
-    /// The size of the brackets, relative to the height of the wrapped content.
-    #[resolve]
-    #[default(Rel::one())]
-    pub size: Rel<Length>,
+    /// The size of the delimiters, relative to the height of the wrapped
+    /// content.
+    ///
+    /// See the [stretch documentation]($math.stretch.size) for more
+    /// information on sizes.
+    #[default(<default_lr_size>::data().into())]
+    pub size: StretchSize,
 
     /// The delimited content, including the delimiters.
     #[required]
@@ -43,9 +56,13 @@ pub struct MidElem {
 /// ```
 #[func]
 pub fn floor(
-    /// The size of the brackets, relative to the height of the wrapped content.
+    /// The size of the delimiters, relative to the height of the wrapped
+    /// content.
+    ///
+    /// See the [stretch documentation]($math.stretch.size) for more
+    /// information on sizes.
     #[named]
-    size: Option<Rel<Length>>,
+    size: Option<StretchSize>,
     /// The expression to floor.
     body: Content,
 ) -> Content {
@@ -59,9 +76,13 @@ pub fn floor(
 /// ```
 #[func]
 pub fn ceil(
-    /// The size of the brackets, relative to the height of the wrapped content.
+    /// The size of the delimiters, relative to the height of the wrapped
+    /// content.
+    ///
+    /// See the [stretch documentation]($math.stretch.size) for more
+    /// information on sizes.
     #[named]
-    size: Option<Rel<Length>>,
+    size: Option<StretchSize>,
     /// The expression to ceil.
     body: Content,
 ) -> Content {
@@ -75,9 +96,13 @@ pub fn ceil(
 /// ```
 #[func]
 pub fn round(
-    /// The size of the brackets, relative to the height of the wrapped content.
+    /// The size of the delimiters, relative to the height of the wrapped
+    /// content.
+    ///
+    /// See the [stretch documentation]($math.stretch.size) for more
+    /// information on sizes.
     #[named]
-    size: Option<Rel<Length>>,
+    size: Option<StretchSize>,
     /// The expression to round.
     body: Content,
 ) -> Content {
@@ -91,9 +116,13 @@ pub fn round(
 /// ```
 #[func]
 pub fn abs(
-    /// The size of the brackets, relative to the height of the wrapped content.
+    /// The size of the delimiters, relative to the height of the wrapped
+    /// content.
+    ///
+    /// See the [stretch documentation]($math.stretch.size) for more
+    /// information on sizes.
     #[named]
-    size: Option<Rel<Length>>,
+    size: Option<StretchSize>,
     /// The expression to take the absolute value of.
     body: Content,
 ) -> Content {
@@ -107,9 +136,13 @@ pub fn abs(
 /// ```
 #[func]
 pub fn norm(
-    /// The size of the brackets, relative to the height of the wrapped content.
+    /// The size of the delimiters, relative to the height of the wrapped
+    /// content.
+    ///
+    /// See the [stretch documentation]($math.stretch.size) for more
+    /// information on sizes.
     #[named]
-    size: Option<Rel<Length>>,
+    size: Option<StretchSize>,
     /// The expression to take the norm of.
     body: Content,
 ) -> Content {
@@ -120,7 +153,7 @@ fn delimited(
     body: Content,
     left: char,
     right: char,
-    size: Option<Rel<Length>>,
+    size: Option<StretchSize>,
 ) -> Content {
     let span = body.span();
     let mut elem = LrElem::new(Content::sequence([

crates/typst-library/src/math/mod.rs

@@ -9,6 +9,7 @@ mod lr;
 mod matrix;
 mod op;
 mod root;
+mod stretch;
 mod style;
 mod underover;
 
@@ -21,6 +22,7 @@ pub use self::lr::*;
 pub use self::matrix::*;
 pub use self::op::*;
 pub use self::root::*;
+pub use self::stretch::*;
 pub use self::style::*;
 pub use self::underover::*;
 

crates/typst-library/src/math/stretch.rs

@@ -0,0 +1,125 @@
+use comemo::Track;
+
+use crate::diag::{At, SourceResult};
+use crate::engine::Engine;
+use crate::foundations::{
+    cast, elem, Content, Context, Func, NativeFunc, NativeFuncData, Resolve, StyleChain,
+};
+use crate::layout::{Abs, Rel};
+use crate::math::{default_lr_size, Mathy};
+
+/// Stretches a glyph.
+///
+/// This function can also be used to automatically stretch the base of an
+/// attachment, so that it fits the top and bottom attachments.
+///
+/// Note that only some glyphs can be stretched, and which ones can depend on
+/// the math font being used. However, most math fonts are the same in this
+/// regard.
+///
+/// ```example
+/// $ H stretch(=)^"define" U + p V $
+/// $ f : X stretch(->>, size: #150%)_"surjective" Y $
+/// $ x stretch(harpoons.ltrb, size: #3em) y
+///     stretch(\[, size: #150%) z $
+/// ```
+#[elem(Mathy)]
+pub struct StretchElem {
+    /// The glyph to stretch.
+    #[required]
+    pub body: Content,
+
+    /// The size to stretch to, relative to the maximum size of the glyph and
+    /// its attachments.
+    ///
+    /// This value can be given as a [relative length]($relative), or a
+    /// [function]($function) that receives the size of the glyph as a
+    /// parameter (an absolute length) and should return a (relative) length.
+    /// For example, `{x => x * 80%}` would be equivalent to just specifying `{80%}`.
+    ///
+    /// Note that the sizes of glyphs in math fonts come about in two ways:
+    ///
+    /// - First, there are pre-made variants at specific sizes. This means you
+    ///   will see discrete jumps in the stretched glyph's size as you increase
+    ///   the size parameter. It is up to the font how many pre-made variants
+    ///   there are and what their sizes are.
+    ///
+    /// - Then, if the pre-made variants are all too small, a glyph of the
+    ///   desired size is assembled from parts. The stretched glyph's size will
+    ///   now be the exact size requested.
+    ///
+    /// It could be the case that only one of the above exist for a glyph in
+    /// the font.
+    ///
+    /// The value given here is really a minimum (but if there is no assembly
+    /// for the glyph, this minimum may not be reached), so the actual size of
+    /// the stretched glyph may not match what you specified.
+    ///
+    /// ```example
+    /// #for i in range(0, 15) {
+    ///   $stretch(\[, size: #(10pt + i * 2pt))$
+    /// }
+    ///
+    /// #set math.stretch(size: x => x + 0.5em)
+    /// $x stretch(=)^"def" y$
+    /// ```
+    #[default(Rel::one().into())]
+    pub size: StretchSize,
+}
+
+/// How to size a stretched glyph.
+#[derive(Debug, Clone, PartialEq, Hash)]
+pub enum StretchSize {
+    /// Sized by the specified length.
+    Rel(Rel),
+    /// Resolve the size for the given base size through the specified
+    /// function.
+    Func(Func),
+}
+
+impl StretchSize {
+    /// Resolve the stretch size given the base size.
+    pub fn resolve(
+        &self,
+        engine: &mut Engine,
+        styles: StyleChain,
+        base: Abs,
+    ) -> SourceResult<Abs> {
+        Ok(match self {
+            Self::Rel(rel) => *rel,
+            Self::Func(func) => func
+                .call(engine, Context::new(None, Some(styles)).track(), [base])?
+                .cast()
+                .at(func.span())?,
+        }
+        .resolve(styles)
+        .relative_to(base))
+    }
+
+    /// Whether the size is the default used by `LrElem`.
+    pub fn is_lr_default(self) -> bool {
+        self == <default_lr_size>::data().into()
+    }
+}
+
+impl From<Rel> for StretchSize {
+    fn from(rel: Rel) -> Self {
+        Self::Rel(rel)
+    }
+}
+
+impl From<&'static NativeFuncData> for StretchSize {
+    fn from(data: &'static NativeFuncData) -> Self {
+        Self::Func(Func::from(data))
+    }
+}
+
+cast! {
+    StretchSize,
+    self => match self {
+        Self::Rel(v) => v.into_value(),
+        Self::Func(v) => v.into_value(),
+    },
+    v: Rel => Self::Rel(v),
+    v: Func => Self::Func(v),
+}

tests/suite/math/delimited.typ

@@ -34,6 +34,11 @@ $ lr(a/b\]) = a = lr(\{a/b) $
 $ lr(]sum_(x=1)^n x], size: #70%)
   < lr((1, 2), size: #200%) $
 
+--- math-lr-size-function ---
+// Test using a function as an argument to size.
+#set math.lr(size: x => if x > 10pt { 1em } else { 4 * x })
+$ (a) (1/2) $
+
 --- math-lr-shorthands ---
 // Test predefined delimiter pairings.
 $floor(x/2), ceil(x/2), abs(x), norm(x)$

tests/suite/math/stretch.typ

@@ -91,3 +91,9 @@ $ body^"text" $
   }
   $body^"long text"$
 }
+
+--- math-stretch-function ---
+// Test using a function as an argument to size.
+$stretch(<-, size: #(x => x - 0.5em))_"function"$
+#set math.stretch(size: x => x + 0.5em)
+$stretch(|) |$