Restructure value conversions 🧱

4 files changed, 222 insertions, 219 deletions

diff --git a/src/eval/args.rs b/src/eval/args.rs
index e9bf378b..765ae09c 100644
--- a/src/eval/args.rs
+++ b/src/eval/args.rs
@@ -1,7 +1,10 @@
 //! Simplifies argument parsing.
 
-use super::{Convert, EvalContext, RefKey, ValueDict};
-use crate::syntax::{SpanWith, Spanned};
+use std::mem;
+
+use super::{Conv, EvalContext, RefKey, TryFromValue, Value, ValueDict};
+use crate::diag::Diag;
+use crate::syntax::{Span, SpanVec, SpanWith, Spanned};
 
 /// A wrapper around a dictionary value that simplifies argument parsing in
 /// functions.
@@ -16,15 +19,11 @@ impl Args {
     pub fn get<'a, K, T>(&mut self, ctx: &mut EvalContext, key: K) -> Option<T>
     where
         K: Into<RefKey<'a>>,
-        T: Convert,
+        T: TryFromValue,
     {
         self.0.v.remove(key).and_then(|entry| {
             let span = entry.value.span;
-            let (result, diag) = T::convert(entry.value);
-            if let Some(diag) = diag {
-                ctx.f.diags.push(diag.span_with(span))
-            }
-            result.ok()
+            conv_diag(T::try_from_value(entry.value), &mut ctx.f.diags, span)
         })
     }
 
@@ -38,37 +37,29 @@ impl Args {
     ) -> Option<T>
     where
         K: Into<RefKey<'a>>,
-        T: Convert,
+        T: TryFromValue,
     {
-        match self.0.v.remove(key) {
-            Some(entry) => {
-                let span = entry.value.span;
-                let (result, diag) = T::convert(entry.value);
-                if let Some(diag) = diag {
-                    ctx.f.diags.push(diag.span_with(span))
-                }
-                result.ok()
-            }
-            None => {
-                ctx.f.diags.push(error!(self.0.span, "missing argument: {}", name));
-                None
-            }
+        if let Some(entry) = self.0.v.remove(key) {
+            let span = entry.value.span;
+            conv_diag(T::try_from_value(entry.value), &mut ctx.f.diags, span)
+        } else {
+            ctx.f.diags.push(error!(self.0.span, "missing argument: {}", name));
+            None
         }
     }
 
     /// Retrieve and remove the first matching positional argument.
     pub fn find<T>(&mut self) -> Option<T>
     where
-        T: Convert,
+        T: TryFromValue,
     {
         for (&key, entry) in self.0.v.nums_mut() {
             let span = entry.value.span;
-            match T::convert(std::mem::take(&mut entry.value)).0 {
-                Ok(t) => {
-                    self.0.v.remove(key);
-                    return Some(t);
-                }
-                Err(v) => entry.value = v.span_with(span),
+            let slot = &mut entry.value;
+            let conv = conv_put_back(T::try_from_value(mem::take(slot)), slot, span);
+            if let Some(t) = conv {
+                self.0.v.remove(key);
+                return Some(t);
             }
         }
         None
@@ -77,18 +68,17 @@ impl Args {
     /// Retrieve and remove all matching positional arguments.
     pub fn find_all<T>(&mut self) -> impl Iterator<Item = T> + '_
     where
-        T: Convert,
+        T: TryFromValue,
     {
         let mut skip = 0;
         std::iter::from_fn(move || {
             for (&key, entry) in self.0.v.nums_mut().skip(skip) {
                 let span = entry.value.span;
-                match T::convert(std::mem::take(&mut entry.value)).0 {
-                    Ok(t) => {
-                        self.0.v.remove(key);
-                        return Some(t);
-                    }
-                    Err(v) => entry.value = v.span_with(span),
+                let slot = &mut entry.value;
+                let conv = conv_put_back(T::try_from_value(mem::take(slot)), slot, span);
+                if let Some(t) = conv {
+                    self.0.v.remove(key);
+                    return Some(t);
                 }
                 skip += 1;
             }
@@ -99,19 +89,18 @@ impl Args {
     /// Retrieve and remove all matching keyword arguments.
     pub fn find_all_str<T>(&mut self) -> impl Iterator<Item = (String, T)> + '_
     where
-        T: Convert,
+        T: TryFromValue,
     {
         let mut skip = 0;
         std::iter::from_fn(move || {
             for (key, entry) in self.0.v.strs_mut().skip(skip) {
                 let span = entry.value.span;
-                match T::convert(std::mem::take(&mut entry.value)).0 {
-                    Ok(t) => {
-                        let key = key.clone();
-                        self.0.v.remove(&key);
-                        return Some((key, t));
-                    }
-                    Err(v) => entry.value = v.span_with(span),
+                let slot = &mut entry.value;
+                let conv = conv_put_back(T::try_from_value(mem::take(slot)), slot, span);
+                if let Some(t) = conv {
+                    let key = key.clone();
+                    self.0.v.remove(&key);
+                    return Some((key, t));
                 }
                 skip += 1;
             }
@@ -129,6 +118,31 @@ impl Args {
     }
 }
 
+fn conv_diag<T>(conv: Conv<T>, diags: &mut SpanVec<Diag>, span: Span) -> Option<T> {
+    match conv {
+        Conv::Ok(t) => Some(t),
+        Conv::Warn(t, warn) => {
+            diags.push(warn.span_with(span));
+            Some(t)
+        }
+        Conv::Err(_, err) => {
+            diags.push(err.span_with(span));
+            None
+        }
+    }
+}
+
+fn conv_put_back<T>(conv: Conv<T>, slot: &mut Spanned<Value>, span: Span) -> Option<T> {
+    match conv {
+        Conv::Ok(t) => Some(t),
+        Conv::Warn(t, _) => Some(t),
+        Conv::Err(v, _) => {
+            *slot = v.span_with(span);
+            None
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::super::{Dict, SpannedEntry, Value};
diff --git a/src/eval/convert.rs b/src/eval/convert.rs
deleted file mode 100644
index 81e42694..00000000
--- a/src/eval/convert.rs
+++ /dev/null
@@ -1,170 +0,0 @@
-//! Conversion from values into other types.
-
-use std::ops::Deref;
-
-use fontdock::{FontStretch, FontStyle, FontWeight};
-
-use super::{Value, ValueDict, ValueFunc};
-use crate::diag::Diag;
-use crate::geom::{Dir, Length, Linear, Relative};
-use crate::paper::Paper;
-use crate::syntax::{Ident, SpanWith, Spanned, SynTree};
-
-/// Types that values can be converted into.
-pub trait Convert: Sized {
-    /// Convert a value into `Self`.
-    ///
-    /// If the conversion works out, this should return `Ok(...)` with an
-    /// instance of `Self`. If it doesn't, it should return `Err(...)` giving
-    /// back the original value.
-    ///
-    /// In addition to the result, the method can return an optional diagnostic
-    /// to warn even when the conversion succeeded or to explain the problem when
-    /// the conversion failed.
-    ///
-    /// The function takes a `Spanned<Value>` instead of just a `Value` so that
-    /// this trait can be blanket implemented for `Spanned<T>` where `T:
-    /// Convert`.
-    fn convert(value: Spanned<Value>) -> (Result<Self, Value>, Option<Diag>);
-}
-
-impl<T: Convert> Convert for Spanned<T> {
-    fn convert(value: Spanned<Value>) -> (Result<Self, Value>, Option<Diag>) {
-        let span = value.span;
-        let (result, diag) = T::convert(value);
-        (result.map(|v| v.span_with(span)), diag)
-    }
-}
-
-macro_rules! convert_match {
-    ($type:ty, $name:expr, $($p:pat => $r:expr),* $(,)?) => {
-        impl $crate::eval::Convert for $type {
-            fn convert(
-                value: $crate::syntax::Spanned<$crate::eval::Value>
-            ) -> (Result<Self, $crate::eval::Value>, Option<$crate::diag::Diag>) {
-                #[allow(unreachable_patterns)]
-                match value.v {
-                    $($p => (Ok($r), None)),*,
-                    v => {
-                        let err = $crate::error!("expected {}, found {}", $name, v.ty());
-                        (Err(v), Some(err))
-                    },
-                }
-            }
-        }
-    };
-}
-
-macro_rules! convert_ident {
-    ($type:ty, $name:expr, $parse:expr) => {
-        impl $crate::eval::Convert for $type {
-            fn convert(
-                value: $crate::syntax::Spanned<$crate::eval::Value>,
-            ) -> (
-                Result<Self, $crate::eval::Value>,
-                Option<$crate::diag::Diag>,
-            ) {
-                match value.v {
-                    Value::Ident(id) => {
-                        if let Some(thing) = $parse(&id) {
-                            (Ok(thing), None)
-                        } else {
-                            (
-                                Err($crate::eval::Value::Ident(id)),
-                                Some($crate::error!("invalid {}", $name)),
-                            )
-                        }
-                    }
-                    v => {
-                        let err = $crate::error!("expected {}, found {}", $name, v.ty());
-                        (Err(v), Some(err))
-                    }
-                }
-            }
-        }
-    };
-}
-
-/// A value type that matches [identifier](Value::Ident) and [string](Value::Str) values.
-pub struct StringLike(pub String);
-
-impl From<StringLike> for String {
-    fn from(like: StringLike) -> String {
-        like.0
-    }
-}
-
-impl Deref for StringLike {
-    type Target = str;
-
-    fn deref(&self) -> &str {
-        self.0.as_str()
-    }
-}
-
-convert_match!(Value, "value", v => v);
-convert_match!(Ident, "identifier", Value::Ident(v) => v);
-convert_match!(bool, "bool", Value::Bool(v) => v);
-convert_match!(i64, "integer", Value::Int(v) => v);
-convert_match!(f64, "float",
-    Value::Int(v) => v as f64,
-    Value::Float(v) => v,
-);
-convert_match!(Length, "length", Value::Length(v) => v);
-convert_match!(Relative, "relative", Value::Relative(v) => v);
-convert_match!(Linear, "linear",
-    Value::Linear(v) => v,
-    Value::Length(v) => v.into(),
-    Value::Relative(v) => v.into(),
-);
-convert_match!(String, "string", Value::Str(v) => v);
-convert_match!(SynTree, "tree", Value::Content(v) => v);
-convert_match!(ValueDict, "dictionary", Value::Dict(v) => v);
-convert_match!(ValueFunc, "function", Value::Func(v) => v);
-convert_match!(StringLike, "identifier or string",
-    Value::Ident(Ident(v)) => StringLike(v),
-    Value::Str(v) => StringLike(v),
-);
-
-convert_ident!(Dir, "direction", |v| match v {
-    "ltr" => Some(Self::LTR),
-    "rtl" => Some(Self::RTL),
-    "ttb" => Some(Self::TTB),
-    "btt" => Some(Self::BTT),
-    _ => None,
-});
-
-convert_ident!(FontStyle, "font style", Self::from_str);
-convert_ident!(FontStretch, "font stretch", Self::from_str);
-convert_ident!(Paper, "paper", Self::from_name);
-
-impl Convert for FontWeight {
-    fn convert(value: Spanned<Value>) -> (Result<Self, Value>, Option<Diag>) {
-        match value.v {
-            Value::Int(number) => {
-                let [min, max] = [100, 900];
-                let warning = if number < min {
-                    Some(warning!("the minimum font weight is {}", min))
-                } else if number > max {
-                    Some(warning!("the maximum font weight is {}", max))
-                } else {
-                    None
-                };
-                let weight = Self::from_number(number.min(max).max(min) as u16);
-                (Ok(weight), warning)
-            }
-            Value::Ident(id) => {
-                if let Some(thing) = FontWeight::from_str(&id) {
-                    (Ok(thing), None)
-                } else {
-                    (Err(Value::Ident(id)), Some(error!("invalid font weight")))
-                }
-            }
-            v => {
-                let err =
-                    error!("expected font weight (name or number), found {}", v.ty());
-                (Err(v), Some(err))
-            }
-        }
-    }
-}
diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index bab93a05..c2b80b68 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -1,15 +1,13 @@
 //! Evaluation of syntax trees.
 
 #[macro_use]
-mod convert;
+mod value;
 mod args;
 mod dict;
 mod scope;
 mod state;
-mod value;
 
 pub use args::*;
-pub use convert::*;
 pub use dict::*;
 pub use scope::*;
 pub use state::*;
diff --git a/src/eval/value.rs b/src/eval/value.rs
index e897fa75..8c98257e 100644
--- a/src/eval/value.rs
+++ b/src/eval/value.rs
@@ -4,10 +4,14 @@ use std::fmt::{self, Debug, Formatter};
 use std::ops::Deref;
 use std::rc::Rc;
 
+use fontdock::{FontStretch, FontStyle, FontWeight};
+
 use super::{Args, Dict, Eval, EvalContext, SpannedEntry};
 use crate::color::RgbaColor;
-use crate::geom::{Length, Linear, Relative};
-use crate::syntax::{Ident, SynTree};
+use crate::diag::Diag;
+use crate::geom::{Dir, Length, Linear, Relative};
+use crate::paper::Paper;
+use crate::syntax::{Ident, SpanWith, Spanned, SynTree};
 
 /// A computational value.
 #[derive(Clone, PartialEq)]
@@ -170,3 +174,160 @@ impl Debug for ValueFunc {
         f.pad("<function>")
     }
 }
+
+/// Try to convert a value into a more specific type.
+pub trait TryFromValue: Sized {
+    /// Try to convert the value into yourself.
+    fn try_from_value(value: Spanned<Value>) -> Conv<Self>;
+}
+
+/// The result of a conversion.
+#[derive(Debug, Clone, PartialEq)]
+pub enum Conv<T> {
+    /// Success conversion.
+    Ok(T),
+    /// Sucessful conversion with a warning.
+    Warn(T, Diag),
+    /// Unsucessful conversion, gives back the value alongside the error.
+    Err(Value, Diag),
+}
+
+impl<T> Conv<T> {
+    /// Map the conversion result.
+    pub fn map<U>(self, f: impl FnOnce(T) -> U) -> Conv<U> {
+        match self {
+            Conv::Ok(t) => Conv::Ok(f(t)),
+            Conv::Warn(t, warn) => Conv::Warn(f(t), warn),
+            Conv::Err(v, err) => Conv::Err(v, err),
+        }
+    }
+}
+
+impl<T: TryFromValue> TryFromValue for Spanned<T> {
+    fn try_from_value(value: Spanned<Value>) -> Conv<Self> {
+        let span = value.span;
+        T::try_from_value(value).map(|v| v.span_with(span))
+    }
+}
+
+/// A value type that matches [identifier](Value::Ident) and [string](Value::Str) values.
+pub struct StringLike(pub String);
+
+impl From<StringLike> for String {
+    fn from(like: StringLike) -> String {
+        like.0
+    }
+}
+
+impl Deref for StringLike {
+    type Target = str;
+
+    fn deref(&self) -> &str {
+        self.0.as_str()
+    }
+}
+
+/// Implement [`TryFromValue`] through a match.
+macro_rules! try_from_match {
+    ($type:ty[$name:literal] $(@ $span:ident)?: $($pattern:pat => $output:expr),* $(,)?) => {
+        impl $crate::eval::TryFromValue for $type {
+            fn try_from_value(value: Spanned<Value>) -> $crate::eval::Conv<Self> {
+                use $crate::eval::Conv;
+                #[allow(unused)]
+                $(let $span = value.span;)?
+                #[allow(unreachable_patterns)]
+                match value.v {
+                    $($pattern => Conv::Ok($output)),*,
+                    v => {
+                        let e = error!("expected {}, found {}", $name, v.ty());
+                        Conv::Err(v, e)
+                    }
+                }
+            }
+        }
+    };
+}
+
+/// Implement [`TryFromValue`] through a function parsing an identifier.
+macro_rules! try_from_id {
+    ($type:ty[$name:literal]: $from_str:expr) => {
+        impl $crate::eval::TryFromValue for $type {
+            fn try_from_value(value: Spanned<Value>) -> $crate::eval::Conv<Self> {
+                use $crate::eval::Conv;
+                let v = value.v;
+                if let Value::Ident(id) = v {
+                    if let Some(v) = $from_str(&id) {
+                        Conv::Ok(v)
+                    } else {
+                        Conv::Err(Value::Ident(id), error!("invalid {}", $name))
+                    }
+                } else {
+                    let e = error!("expected identifier, found {}", v.ty());
+                    Conv::Err(v, e)
+                }
+            }
+        }
+    };
+}
+
+try_from_match!(Value["value"]: v => v);
+try_from_match!(Ident["identifier"]: Value::Ident(v) => v);
+try_from_match!(bool["bool"]: Value::Bool(v) => v);
+try_from_match!(i64["integer"]: Value::Int(v) => v);
+try_from_match!(f64["float"]:
+    Value::Int(v) => v as f64,
+    Value::Float(v) => v,
+);
+try_from_match!(Length["length"]: Value::Length(v) => v);
+try_from_match!(Relative["relative"]: Value::Relative(v) => v);
+try_from_match!(Linear["linear"]:
+    Value::Linear(v) => v,
+    Value::Length(v) => v.into(),
+    Value::Relative(v) => v.into(),
+);
+try_from_match!(String["string"]: Value::Str(v) => v);
+try_from_match!(SynTree["tree"]: Value::Content(v) => v);
+try_from_match!(ValueDict["dictionary"]: Value::Dict(v) => v);
+try_from_match!(ValueFunc["function"]: Value::Func(v) => v);
+try_from_match!(StringLike["identifier or string"]:
+    Value::Ident(Ident(v)) => Self(v),
+    Value::Str(v) => Self(v),
+);
+try_from_id!(Dir["direction"]: |v| match v {
+    "ltr" | "left-to-right" => Some(Self::LTR),
+    "rtl" | "right-to-left" => Some(Self::RTL),
+    "ttb" | "top-to-bottom" => Some(Self::TTB),
+    "btt" | "bottom-to-top" => Some(Self::BTT),
+    _ => None,
+});
+try_from_id!(FontStyle["font style"]: Self::from_str);
+try_from_id!(FontStretch["font stretch"]: Self::from_str);
+try_from_id!(Paper["paper"]: Self::from_name);
+
+impl TryFromValue for FontWeight {
+    fn try_from_value(value: Spanned<Value>) -> Conv<Self> {
+        match value.v {
+            Value::Int(number) => {
+                let [min, max] = [Self::THIN, Self::BLACK];
+                if number < i64::from(min.to_number()) {
+                    Conv::Warn(min, warning!("the minimum font weight is {:#?}", min))
+                } else if number > i64::from(max.to_number()) {
+                    Conv::Warn(max, warning!("the maximum font weight is {:#?}", max))
+                } else {
+                    Conv::Ok(Self::from_number(number as u16))
+                }
+            }
+            Value::Ident(id) => {
+                if let Some(weight) = Self::from_str(&id) {
+                    Conv::Ok(weight)
+                } else {
+                    Conv::Err(Value::Ident(id), error!("invalid font weight"))
+                }
+            }
+            v => {
+                let e = error!("expected font weight, found {}", v.ty());
+                Conv::Err(v, e)
+            }
+        }
+    }
+}