feat(tvix/eval): add module for string type implementation

Change-Id: I5e4465acc4a676c10d7374b14f7a09240202b466
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6085
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/value/mod.rs

@@ -0,0 +1,56 @@
+//! This module implements the backing representation of runtime
+//! values in the Nix language.
+use std::fmt::Display;
+
+use crate::errors::{Error, EvalResult};
+
+mod string;
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum Value {
+    Null,
+    Bool(bool),
+    Integer(i64),
+    Float(f64),
+}
+
+impl Value {
+    pub fn is_number(&self) -> bool {
+        match self {
+            Value::Integer(_) => true,
+            Value::Float(_) => true,
+            _ => false,
+        }
+    }
+
+    pub fn type_of(&self) -> &'static str {
+        match self {
+            Value::Null => "null",
+            Value::Bool(_) => "bool",
+            Value::Integer(_) => "int",
+            Value::Float(_) => "float",
+        }
+    }
+
+    pub fn as_bool(self) -> EvalResult<bool> {
+        match self {
+            Value::Bool(b) => Ok(b),
+            other => Err(Error::TypeError {
+                expected: "bool",
+                actual: other.type_of(),
+            }),
+        }
+    }
+}
+
+impl Display for Value {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Value::Null => f.write_str("null"),
+            Value::Bool(true) => f.write_str("true"),
+            Value::Bool(false) => f.write_str("false"),
+            Value::Integer(num) => f.write_fmt(format_args!("{}", num)),
+            Value::Float(num) => f.write_fmt(format_args!("{}", num)),
+        }
+    }
+}