Move casting into separate module

1 files changed, 358 insertions, 0 deletions

diff --git a/src/eval/cast.rs b/src/eval/cast.rs
new file mode 100644
index 00000000..063d378a
--- /dev/null
+++ b/src/eval/cast.rs
@@ -0,0 +1,358 @@
+use std::num::NonZeroUsize;
+
+use super::{Regex, Value};
+use crate::diag::{with_alternative, StrResult};
+use crate::geom::{Corners, Dir, Paint, Sides};
+use crate::model::{Content, Group, Layout, LayoutNode, Pattern};
+use crate::syntax::Spanned;
+
+/// Cast from a value to a specific type.
+pub trait Cast<V = Value>: Sized {
+    /// Check whether the value is castable to `Self`.
+    fn is(value: &V) -> bool;
+
+    /// Try to cast the value into an instance of `Self`.
+    fn cast(value: V) -> StrResult<Self>;
+}
+
+/// Implement traits for dynamic types.
+macro_rules! dynamic {
+    ($type:ty: $name:literal, $($tts:tt)*) => {
+        impl $crate::eval::Type for $type {
+            const TYPE_NAME: &'static str = $name;
+        }
+
+        castable! {
+            $type,
+            Expected: <Self as $crate::eval::Type>::TYPE_NAME,
+            $($tts)*
+            @this: Self => this.clone(),
+        }
+
+        impl From<$type> for $crate::eval::Value {
+            fn from(v: $type) -> Self {
+                $crate::eval::Value::Dyn($crate::eval::Dynamic::new(v))
+            }
+        }
+    };
+}
+
+/// Make a type castable from a value.
+macro_rules! castable {
+    ($type:ty: $inner:ty) => {
+        impl $crate::eval::Cast<$crate::eval::Value> for $type {
+            fn is(value: &$crate::eval::Value) -> bool {
+                <$inner>::is(value)
+            }
+
+            fn cast(value: $crate::eval::Value) -> $crate::diag::StrResult<Self> {
+                <$inner>::cast(value).map(Self)
+            }
+        }
+    };
+
+    (
+        $type:ty,
+        Expected: $expected:expr,
+        $($pattern:pat => $out:expr,)*
+        $(@$dyn_in:ident: $dyn_type:ty => $dyn_out:expr,)*
+    ) => {
+        #[allow(unreachable_patterns)]
+        impl $crate::eval::Cast<$crate::eval::Value> for $type {
+            fn is(value: &$crate::eval::Value) -> bool {
+                #[allow(unused_variables)]
+                match value {
+                    $($pattern => true,)*
+                    $crate::eval::Value::Dyn(dynamic) => {
+                        false $(|| dynamic.is::<$dyn_type>())*
+                    }
+                    _ => false,
+                }
+            }
+
+            fn cast(value: $crate::eval::Value) -> $crate::diag::StrResult<Self> {
+                let found = match value {
+                    $($pattern => return Ok($out),)*
+                    $crate::eval::Value::Dyn(dynamic) => {
+                        $(if let Some($dyn_in) = dynamic.downcast::<$dyn_type>() {
+                            return Ok($dyn_out);
+                        })*
+                        dynamic.type_name()
+                    }
+                    v => v.type_name(),
+                };
+
+                Err(format!("expected {}, found {}", $expected, found))
+            }
+        }
+    };
+}
+
+impl Cast for Value {
+    fn is(_: &Value) -> bool {
+        true
+    }
+
+    fn cast(value: Value) -> StrResult<Self> {
+        Ok(value)
+    }
+}
+
+impl<T: Cast> Cast<Spanned<Value>> for T {
+    fn is(value: &Spanned<Value>) -> bool {
+        T::is(&value.v)
+    }
+
+    fn cast(value: Spanned<Value>) -> StrResult<Self> {
+        T::cast(value.v)
+    }
+}
+
+impl<T: Cast> Cast<Spanned<Value>> for Spanned<T> {
+    fn is(value: &Spanned<Value>) -> bool {
+        T::is(&value.v)
+    }
+
+    fn cast(value: Spanned<Value>) -> StrResult<Self> {
+        let span = value.span;
+        T::cast(value.v).map(|t| Spanned::new(t, span))
+    }
+}
+
+dynamic! {
+    Dir: "direction",
+}
+
+dynamic! {
+    Regex: "regular expression",
+}
+
+dynamic! {
+    Group: "group",
+}
+
+castable! {
+    usize,
+    Expected: "non-negative integer",
+    Value::Int(int) => int.try_into().map_err(|_| {
+        if int < 0 {
+            "must be at least zero"
+        } else {
+            "number too large"
+        }
+    })?,
+}
+
+castable! {
+    NonZeroUsize,
+    Expected: "positive integer",
+    Value::Int(int) => int
+        .try_into()
+        .and_then(|int: usize| int.try_into())
+        .map_err(|_| if int <= 0 {
+            "must be positive"
+        } else {
+            "number too large"
+        })?,
+}
+
+castable! {
+    Paint,
+    Expected: "color",
+    Value::Color(color) => Paint::Solid(color),
+}
+
+castable! {
+    String,
+    Expected: "string",
+    Value::Str(string) => string.into(),
+}
+
+castable! {
+    LayoutNode,
+    Expected: "content",
+    Value::None => Self::default(),
+    Value::Str(text) => Content::Text(text.into()).pack(),
+    Value::Content(content) => content.pack(),
+}
+
+castable! {
+    Pattern,
+    Expected: "function, string or regular expression",
+    Value::Func(func) => Self::Node(func.node()?),
+    Value::Str(text) => Self::text(&text),
+    @regex: Regex => Self::Regex(regex.clone()),
+}
+
+impl<T: Cast> Cast for Option<T> {
+    fn is(value: &Value) -> bool {
+        matches!(value, Value::None) || T::is(value)
+    }
+
+    fn cast(value: Value) -> StrResult<Self> {
+        match value {
+            Value::None => Ok(None),
+            v => T::cast(v).map(Some).map_err(|msg| with_alternative(msg, "none")),
+        }
+    }
+}
+
+/// A value that can be automatically determined.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+pub enum Smart<T> {
+    /// The value should be determined smartly based on the circumstances.
+    Auto,
+    /// A specific value.
+    Custom(T),
+}
+
+impl<T> Smart<T> {
+    /// Map the contained custom value with `f`.
+    pub fn map<F, U>(self, f: F) -> Smart<U>
+    where
+        F: FnOnce(T) -> U,
+    {
+        match self {
+            Self::Auto => Smart::Auto,
+            Self::Custom(x) => Smart::Custom(f(x)),
+        }
+    }
+
+    /// Keeps `self` if it contains a custom value, otherwise returns `other`.
+    pub fn or(self, other: Smart<T>) -> Self {
+        match self {
+            Self::Custom(x) => Self::Custom(x),
+            Self::Auto => other,
+        }
+    }
+
+    /// Returns the contained custom value or a provided default value.
+    pub fn unwrap_or(self, default: T) -> T {
+        match self {
+            Self::Auto => default,
+            Self::Custom(x) => x,
+        }
+    }
+
+    /// Returns the contained custom value or computes a default value.
+    pub fn unwrap_or_else<F>(self, f: F) -> T
+    where
+        F: FnOnce() -> T,
+    {
+        match self {
+            Self::Auto => f(),
+            Self::Custom(x) => x,
+        }
+    }
+
+    /// Returns the contained custom value or the default value.
+    pub fn unwrap_or_default(self) -> T
+    where
+        T: Default,
+    {
+        self.unwrap_or_else(T::default)
+    }
+}
+
+impl<T> Default for Smart<T> {
+    fn default() -> Self {
+        Self::Auto
+    }
+}
+
+impl<T: Cast> Cast for Smart<T> {
+    fn is(value: &Value) -> bool {
+        matches!(value, Value::Auto) || T::is(value)
+    }
+
+    fn cast(value: Value) -> StrResult<Self> {
+        match value {
+            Value::Auto => Ok(Self::Auto),
+            v => T::cast(v)
+                .map(Self::Custom)
+                .map_err(|msg| with_alternative(msg, "auto")),
+        }
+    }
+}
+
+impl<T> Cast for Sides<T>
+where
+    T: Cast + Default + Copy,
+{
+    fn is(value: &Value) -> bool {
+        matches!(value, Value::Dict(_)) || T::is(value)
+    }
+
+    fn cast(mut value: Value) -> StrResult<Self> {
+        if let Value::Dict(dict) = &mut value {
+            let mut take = |key| dict.take(key).map(T::cast).transpose();
+
+            let rest = take("rest")?;
+            let x = take("x")?.or(rest);
+            let y = take("y")?.or(rest);
+            let sides = Sides {
+                left: take("left")?.or(x),
+                top: take("top")?.or(y),
+                right: take("right")?.or(x),
+                bottom: take("bottom")?.or(y),
+            };
+
+            if let Some((key, _)) = dict.iter().next() {
+                return Err(format!("unexpected key {key:?}"));
+            }
+
+            Ok(sides.map(Option::unwrap_or_default))
+        } else {
+            T::cast(value).map(Self::splat).map_err(|msg| {
+                with_alternative(
+                    msg,
+                    "dictionary with any of \
+                     `left`, `top`, `right`, `bottom`, \
+                     `x`, `y`, or `rest` as keys",
+                )
+            })
+        }
+    }
+}
+
+impl<T> Cast for Corners<T>
+where
+    T: Cast + Default + Copy,
+{
+    fn is(value: &Value) -> bool {
+        matches!(value, Value::Dict(_)) || T::is(value)
+    }
+
+    fn cast(mut value: Value) -> StrResult<Self> {
+        if let Value::Dict(dict) = &mut value {
+            let mut take = |key| dict.take(key).map(T::cast).transpose();
+
+            let rest = take("rest")?;
+            let left = take("left")?.or(rest);
+            let top = take("top")?.or(rest);
+            let right = take("right")?.or(rest);
+            let bottom = take("bottom")?.or(rest);
+            let corners = Corners {
+                top_left: take("top-left")?.or(top).or(left),
+                top_right: take("top-right")?.or(top).or(right),
+                bottom_right: take("bottom-right")?.or(bottom).or(right),
+                bottom_left: take("bottom-left")?.or(bottom).or(left),
+            };
+
+            if let Some((key, _)) = dict.iter().next() {
+                return Err(format!("unexpected key {key:?}"));
+            }
+
+            Ok(corners.map(Option::unwrap_or_default))
+        } else {
+            T::cast(value).map(Self::splat).map_err(|msg| {
+                with_alternative(
+                    msg,
+                    "dictionary with any of \
+                     `top-left`, `top-right`, `bottom-right`, `bottom-left`, \
+                     `left`, `top`, `right`, `bottom`, or `rest` as keys",
+                )
+            })
+        }
+    }
+}