Lock down numbers

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/builtins/strings.rs

@@ -0,0 +1,19 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+use crate::value::Value;
+
+use anyhow::{bail, Result};
+
+fn ensure_numeric(fcn: &str, arg: &Expr, v: &Value) -> Result<Float> {
+    Ok(match &v {
+        Value::Number(n) => n.0 .0,
+        _ => {
+            let span = arg.span();
+            bail!(
+                span.error(format!("`{fcn}` expects numeric argument. Got `{v}` instead").as_str())
+            )
+        }
+    })
+}
+