diff --git a/src/config.rs b/src/config.rs
index 365970e..36cc82c 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -195,10 +195,11 @@ mod expression {
     named!(cpu_temperature => "$cpu-temperature");
     named!(cpu_temperature_volatility => "$cpu-temperature-volatility");
     named!(cpu_idle_seconds => "$cpu-idle-seconds");
+
     named!(power_supply_charge => "%power-supply-charge");
     named!(power_supply_discharge_rate => "%power-supply-discharge-rate");
-    named!(charging => "?charging");
-    named!(on_battery => "?on-battery");
+
+    named!(discharging => "?discharging");
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq)]
@@ -225,65 +226,80 @@ pub enum Expression {
     #[serde(with = "expression::power_supply_discharge_rate")]
     PowerSupplyDischargeRate,
 
-    #[serde(with = "expression::charging")]
-    Charging,
-    #[serde(with = "expression::on_battery")]
-    OnBattery,
+    #[serde(with = "expression::discharging")]
+    Discharging,
 
     Boolean(bool),
 
     Number(f64),
 
     Plus {
-        value: Box<Expression>,
-        plus: Box<Expression>,
+        #[serde(rename = "value")]
+        a: Box<Expression>,
+        #[serde(rename = "plus")]
+        b: Box<Expression>,
     },
     Minus {
-        value: Box<Expression>,
-        minus: Box<Expression>,
+        #[serde(rename = "value")]
+        a: Box<Expression>,
+        #[serde(rename = "minus")]
+        b: Box<Expression>,
     },
     Multiply {
-        value: Box<Expression>,
-        multiply: Box<Expression>,
+        #[serde(rename = "value")]
+        a: Box<Expression>,
+        #[serde(rename = "multiply")]
+        b: Box<Expression>,
     },
     Power {
-        value: Box<Expression>,
-        power: Box<Expression>,
+        #[serde(rename = "value")]
+        a: Box<Expression>,
+        #[serde(rename = "power")]
+        b: Box<Expression>,
     },
     Divide {
-        value: Box<Expression>,
-        divide: Box<Expression>,
+        #[serde(rename = "value")]
+        a: Box<Expression>,
+        #[serde(rename = "divide")]
+        b: Box<Expression>,
     },
 
-    #[serde(rename_all = "kebab-case")]
     LessThan {
-        value: Box<Expression>,
-        is_less_than: Box<Expression>,
+        #[serde(rename = "value")]
+        a: Box<Expression>,
+        #[serde(rename = "is-less-than")]
+        b: Box<Expression>,
     },
-    #[serde(rename_all = "kebab-case")]
     MoreThan {
-        value: Box<Expression>,
-        is_more_than: Box<Expression>,
+        #[serde(rename = "value")]
+        a: Box<Expression>,
+        #[serde(rename = "is-more-than")]
+        b: Box<Expression>,
     },
 
-    #[serde(rename_all = "kebab-case")]
     Equal {
-        value: Box<Expression>,
-        is_equal: Box<Expression>,
+        #[serde(rename = "value")]
+        a: Box<Expression>,
+        #[serde(rename = "is-equal")]
+        b: Box<Expression>,
         leeway: Box<Expression>,
     },
 
     And {
-        value: Box<Expression>,
-        and: Box<Expression>,
+        #[serde(rename = "value")]
+        a: Box<Expression>,
+        #[serde(rename = "and")]
+        b: Box<Expression>,
     },
     All {
         all: Vec<Expression>,
     },
 
     Or {
-        value: Box<Expression>,
-        or: Box<Expression>,
+        #[serde(rename = "value")]
+        a: Box<Expression>,
+        #[serde(rename = "or")]
+        b: Box<Expression>,
     },
     Any {
         any: Vec<Expression>,
@@ -318,6 +334,121 @@ impl Expression {
     }
 }
 
+#[derive(Debug, Clone, PartialEq)]
+pub struct EvalState {
+    pub cpu_usage: f64,
+    pub cpu_usage_volatility: Option<f64>,
+    pub cpu_temperature: f64,
+    pub cpu_temperature_volatility: Option<f64>,
+    pub cpu_idle_seconds: f64,
+
+    pub power_supply_charge: f64,
+    pub power_supply_discharge_rate: Option<f64>,
+
+    pub discharging: bool,
+}
+
+impl Expression {
+    pub fn eval(&self, state: &EvalState) -> anyhow::Result<Option<Expression>> {
+        use Expression::*;
+
+        macro_rules! try_ok {
+            ($expression:expr) => {
+                match $expression {
+                    Some(value) => value,
+                    None => return Ok(None),
+                }
+            };
+        }
+
+        macro_rules! eval {
+            ($expression:expr) => {
+                try_ok!($expression.eval(state)?)
+            };
+        }
+
+        // [e8dax09]: This may be look inefficient, and it definitely isn't optimal,
+        // but expressions in rules are usually so small that it doesn't matter or
+        // make a perceiveable performance difference.
+        //
+        // We also want to be strict, instead of lazy in binary operations, because
+        // we want to catch type errors immediately.
+        //
+        // FIXME: We currently cannot catch errors that will happen when propagating None.
+        // You can have a type error go uncaught on first startup by using $cpu-usage-volatility
+        // incorrectly, for example.
+        Ok(Some(match self {
+            CpuUsage => Number(state.cpu_usage),
+            CpuUsageVolatility => Number(try_ok!(state.cpu_usage_volatility)),
+            CpuTemperature => Number(state.cpu_temperature),
+            CpuTemperatureVolatility => Number(try_ok!(state.cpu_temperature_volatility)),
+            CpuIdleSeconds => Number(state.cpu_idle_seconds),
+
+            PowerSupplyCharge => Number(state.cpu_idle_seconds),
+            PowerSupplyDischargeRate => Number(try_ok!(state.power_supply_discharge_rate)),
+
+            Discharging => Boolean(state.discharging),
+
+            literal @ (Boolean(_) | Number(_)) => literal.clone(),
+
+            Plus { a, b } => Number(eval!(a).as_number()? + eval!(b).as_number()?),
+            Minus { a, b } => Number(eval!(a).as_number()? - eval!(b).as_number()?),
+            Multiply { a, b } => Number(eval!(a).as_number()? * eval!(b).as_number()?),
+            Power { a, b } => Number(eval!(a).as_number()?.powf(eval!(b).as_number()?)),
+            Divide { a, b } => Number(eval!(a).as_number()? / eval!(b).as_number()?),
+
+            LessThan { a, b } => Boolean(eval!(a).as_number()? < eval!(b).as_number()?),
+            MoreThan { a, b } => Boolean(eval!(a).as_number()? > eval!(b).as_number()?),
+            Equal { a, b, leeway } => {
+                let a = eval!(a).as_number()?;
+                let b = eval!(b).as_number()?;
+                let leeway = eval!(leeway).as_number()?;
+
+                let minimum = a - leeway;
+                let maximum = a + leeway;
+
+                Boolean(minimum < b && b < maximum)
+            }
+
+            And { a, b } => {
+                let a = eval!(a).as_boolean()?;
+                let b = eval!(b).as_boolean()?;
+
+                Boolean(a && b)
+            }
+            All { all } => {
+                let mut result = true;
+
+                for value in all {
+                    let value = eval!(value).as_boolean()?;
+
+                    result = result && value;
+                }
+
+                Boolean(result)
+            }
+            Or { a, b } => {
+                let a = eval!(a).as_boolean()?;
+                let b = eval!(b).as_boolean()?;
+
+                Boolean(a || b)
+            }
+            Any { any } => {
+                let mut result = false;
+
+                for value in any {
+                    let value = eval!(value).as_boolean()?;
+
+                    result = result || value;
+                }
+
+                Boolean(result)
+            }
+            Not { not } => Boolean(!eval!(not).as_boolean()?),
+        }))
+    }
+}
+
 #[derive(Serialize, Deserialize, Default, Debug, Clone, PartialEq)]
 #[serde(deny_unknown_fields, rename_all = "kebab-case")]
 pub struct Rule {
diff --git a/src/daemon.rs b/src/daemon.rs
index fc1a6a5..43cac22 100644
--- a/src/daemon.rs
+++ b/src/daemon.rs
@@ -306,121 +306,6 @@ impl Daemon {
     }
 }
 
-impl Daemon {
-    fn eval(&self, expression: &config::Expression) -> anyhow::Result<Option<config::Expression>> {
-        use config::Expression::*;
-
-        macro_rules! try_ok {
-            ($expression:expr) => {
-                match $expression {
-                    Some(value) => value,
-                    None => return Ok(None),
-                }
-            };
-        }
-
-        Ok(Some(match expression {
-            CpuUsage => Number(self.cpu_log.back().unwrap().usage),
-            CpuUsageVolatility => Number(try_ok!(self.cpu_volatility()).usage),
-            CpuTemperature => Number(self.cpu_log.back().unwrap().temperature),
-            CpuTemperatureVolatility => Number(try_ok!(self.cpu_volatility()).temperature),
-            CpuIdleSeconds => Number(self.last_user_activity.elapsed().as_secs_f64()),
-            PowerSupplyCharge => Number(self.power_supply_log.back().unwrap().charge),
-            PowerSupplyDischargeRate => Number(try_ok!(self.power_supply_discharge_rate())),
-
-            Charging => Boolean(!self.discharging()),
-            OnBattery => Boolean(self.discharging()),
-
-            literal @ Boolean(_) | literal @ Number(_) => literal.clone(),
-
-            Plus { value, plus } => Number(
-                try_ok!(self.eval(value)?).as_number()? + try_ok!(self.eval(plus)?).as_number()?,
-            ),
-            Minus { value, minus } => Number(
-                try_ok!(self.eval(value)?).as_number()? - try_ok!(self.eval(minus)?).as_number()?,
-            ),
-            Multiply { value, multiply } => Number(
-                try_ok!(self.eval(value)?).as_number()?
-                    * try_ok!(self.eval(multiply)?).as_number()?,
-            ),
-            Power { value, power } => Number(
-                try_ok!(self.eval(value)?)
-                    .as_number()?
-                    .powf(try_ok!(self.eval(power)?).as_number()?),
-            ),
-            Divide { value, divide } => Number(
-                try_ok!(self.eval(value)?).as_number()?
-                    / try_ok!(self.eval(divide)?).as_number()?,
-            ),
-
-            LessThan {
-                value,
-                is_less_than,
-            } => Boolean(
-                try_ok!(self.eval(value)?).as_number()?
-                    < try_ok!(self.eval(is_less_than)?).as_number()?,
-            ),
-            MoreThan {
-                value,
-                is_more_than,
-            } => Boolean(
-                try_ok!(self.eval(value)?).as_number()?
-                    > try_ok!(self.eval(is_more_than)?).as_number()?,
-            ),
-            Equal {
-                value,
-                is_equal,
-                leeway,
-            } => {
-                let value = try_ok!(self.eval(value)?).as_number()?;
-                let leeway = try_ok!(self.eval(leeway)?).as_number()?;
-
-                let is_equal = try_ok!(self.eval(is_equal)?).as_number()?;
-
-                let minimum = value - leeway;
-                let maximum = value + leeway;
-
-                Boolean(minimum < is_equal && is_equal < maximum)
-            }
-
-            And { value, and } => Boolean(
-                try_ok!(self.eval(value)?).as_boolean()?
-                    && try_ok!(self.eval(and)?).as_boolean()?,
-            ),
-            All { all } => {
-                let mut result = true;
-
-                for value in all {
-                    result = result && try_ok!(self.eval(value)?).as_boolean()?;
-
-                    if !result {
-                        break;
-                    }
-                }
-
-                Boolean(result)
-            }
-            Or { value, or } => Boolean(
-                try_ok!(self.eval(value)?).as_boolean()? || try_ok!(self.eval(or)?).as_boolean()?,
-            ),
-            Any { any } => {
-                let mut result = false;
-
-                for value in any {
-                    result = result || try_ok!(self.eval(value)?).as_boolean()?;
-
-                    if result {
-                        break;
-                    }
-                }
-
-                Boolean(result)
-            }
-            Not { not } => Boolean(!try_ok!(self.eval(not)?).as_boolean()?),
-        }))
-    }
-}
-
 pub fn run(config: config::DaemonConfig) -> anyhow::Result<()> {
     assert!(config.rules.is_sorted_by_key(|rule| rule.priority));
 
@@ -451,8 +336,19 @@ pub fn run(config: config::DaemonConfig) -> anyhow::Result<()> {
 
         let sleep_until = Instant::now() + daemon.polling_interval();
 
+        let state = config::EvalState {
+            cpu_usage: daemon.cpu_log.back().unwrap().usage,
+            cpu_usage_volatility: daemon.cpu_volatility().map(|vol| vol.usage),
+            cpu_temperature: daemon.cpu_log.back().unwrap().temperature,
+            cpu_temperature_volatility: daemon.cpu_volatility().map(|vol| vol.temperature),
+            cpu_idle_seconds: daemon.last_user_activity.elapsed().as_secs_f64(),
+            power_supply_charge: daemon.power_supply_log.back().unwrap().charge,
+            power_supply_discharge_rate: daemon.power_supply_discharge_rate(),
+            discharging: daemon.discharging(),
+        };
+
         for rule in &config.rules {
-            let Some(condition) = daemon.eval(&rule.if_)? else {
+            let Some(condition) = rule.if_.eval(&state)? else {
                 continue;
             };