Add natural logarithm (#7258)

- `math ln`

crates/nu-command/src/default_context.rs

@@ -419,6 +419,7 @@ pub fn create_default_context() -> EngineState {
             MathArcTanH,
             MathPi,
             MathEuler,
+            MathLn,
         };
 
         // Network

crates/nu-command/src/math/ln.rs

@@ -0,0 +1,95 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{Category, Example, PipelineData, ShellError, Signature, Span, Type, Value};
+
+#[derive(Clone)]
+pub struct SubCommand;
+
+impl Command for SubCommand {
+    fn name(&self) -> &str {
+        "math ln"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("math ln")
+            .input_output_types(vec![(Type::Number, Type::Float)])
+            .vectorizes_over_list(true)
+            .category(Category::Math)
+    }
+
+    fn usage(&self) -> &str {
+        "Returns the natural logarithm. Base: (math e)"
+    }
+
+    fn search_terms(&self) -> Vec<&str> {
+        vec!["natural", "logarithm", "inverse", "euler"]
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        _stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
+        let head = call.head;
+        input.map(
+            move |value| operate(value, head),
+            engine_state.ctrlc.clone(),
+        )
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Get the natural logarithm of e",
+            example: "math e | math ln",
+            result: Some(Value::test_float(1.0f64)),
+        }]
+    }
+}
+
+fn operate(value: Value, head: Span) -> Value {
+    match value {
+        numeric @ (Value::Int { .. } | Value::Float { .. }) => {
+            let (val, span) = match numeric {
+                Value::Int { val, span } => (val as f64, span),
+                Value::Float { val, span } => (val, span),
+                _ => unreachable!(),
+            };
+
+            if val > 0.0 {
+                let val = val.ln();
+
+                Value::Float { val, span }
+            } else {
+                Value::Error {
+                    error: ShellError::UnsupportedInput(
+                        "'ln' undefined for values outside the open interval (0, Inf).".into(),
+                        span,
+                    ),
+                }
+            }
+        }
+        other => Value::Error {
+            error: ShellError::UnsupportedInput(
+                format!(
+                    "Only numerical values are supported, input type: {:?}",
+                    other.get_type()
+                ),
+                other.span().unwrap_or(head),
+            ),
+        },
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(SubCommand {})
+    }
+}

crates/nu-command/src/math/mod.rs

@@ -12,6 +12,7 @@ mod cosh;
 mod euler;
 mod eval;
 mod floor;
+mod ln;
 pub mod math_;
 mod max;
 mod median;
@@ -64,3 +65,5 @@ pub use arctanh::SubCommand as MathArcTanH;
 
 pub use euler::SubCommand as MathEuler;
 pub use pi::SubCommand as MathPi;
+
+pub use ln::SubCommand as MathLn;