diff --git a/src/function/eval.rs b/src/function/eval.rs
new file mode 100644
index 0000000..077e075
--- /dev/null
+++ b/src/function/eval.rs
@@ -0,0 +1,81 @@
+use ::Error;
+use super::Function;
+
+impl Function {
+    pub fn eval(&self) -> Result<bool, Error> {
+        // TODO: Handle all variants.
+        // TODO: Paths may not lead outside game directory.
+        match *self {
+            Function::FilePath(ref f) => Ok(f.exists()),
+            _ => Ok(false),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use function::Function;
+
+    use std::path::PathBuf;
+
+    #[test]
+    fn function_file_path_eval_should_return_true_if_the_file_exists_relative_to_the_data_path() {
+        let function = Function::FilePath(PathBuf::from("Cargo.toml"));
+
+        assert!(function.eval().unwrap());
+
+        unimplemented!("not yet any way to actually specify the data path");
+    }
+
+    #[test]
+    fn function_file_path_eval_should_return_true_if_given_a_plugin_that_is_ghosted() {
+        let function = Function::FilePath(PathBuf::from("test.esp"));
+
+        assert!(function.eval().unwrap());
+
+        unimplemented!("need to add tempdir and create a test.esp.ghost");
+    }
+
+    #[test]
+    #[allow(non_snake_case)]
+    fn function_file_path_eval_should_be_true_if_given_LOOT() {
+        unimplemented!();
+    }
+
+    #[test]
+    fn function_file_path_eval_should_not_check_for_ghosted_non_plugin_file() {
+        unimplemented!();
+    }
+
+    #[test]
+    fn function_file_path_eval_should_error_if_the_path_is_outside_game_directory() {
+        unimplemented!("to do");
+    }
+
+    #[test]
+    fn function_file_path_eval_should_return_false_if_the_file_does_not_exist() {
+        let function = Function::FilePath(PathBuf::from("missing"));
+
+        assert!(!function.eval().unwrap());
+    }
+
+    #[test]
+    fn function_file_regex_eval_should_error_if_the_path_is_outside_game_directory() {
+        unimplemented!();
+    }
+
+    #[test]
+    fn function_file_regex_eval_should_be_false_if_no_directory_entries_match() {
+        unimplemented!();
+    }
+
+    #[test]
+    fn function_file_regex_eval_should_be_false_if_the_parent_path_part_is_not_a_directory() {
+        unimplemented!();
+    }
+
+    #[test]
+    fn function_file_regex_eval_should_be_true_if_a_directory_entry_matches() {
+        unimplemented!();
+    }
+}
diff --git a/src/function/mod.rs b/src/function/mod.rs
new file mode 100644
index 0000000..b0fe44b
--- /dev/null
+++ b/src/function/mod.rs
@@ -0,0 +1,28 @@
+use std::path::PathBuf;
+
+use regex::Regex;
+
+pub mod eval;
+pub mod parse;
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum ComparisonOperator {
+    Equal,
+    NotEqual,
+    LessThan,
+    GreaterThan,
+    LessThanOrEqual,
+    GreaterThanOrEqual,
+}
+
+#[derive(Debug)]
+pub enum Function {
+    FilePath(PathBuf),
+    FileRegex(Regex),
+    ActivePath(PathBuf),
+    ActiveRegex(Regex),
+    Many(Regex),
+    ManyActive(Regex),
+    Checksum(PathBuf, u32),
+    Version(PathBuf, String, ComparisonOperator),
+}
diff --git a/src/function.rs b/src/function/parse.rs
similarity index 74%
rename from src/function.rs
rename to src/function/parse.rs
index 3bfb485..a4aeb4a 100644
--- a/src/function.rs
+++ b/src/function/parse.rs
@@ -4,17 +4,7 @@ use std::str;
 use nom::{hex_digit, Context, Err, ErrorKind, IResult};
 use regex::Regex;
 
-use super::Error;
-
-#[derive(Debug, PartialEq, Eq)]
-pub enum ComparisonOperator {
-    Equal,
-    NotEqual,
-    LessThan,
-    GreaterThan,
-    LessThanOrEqual,
-    GreaterThanOrEqual,
-}
+use super::{ComparisonOperator, Function};
 
 impl ComparisonOperator {
     pub fn parse(input: &str) -> IResult<&str, ComparisonOperator> {
@@ -33,18 +23,6 @@ impl ComparisonOperator {
     }
 }
 
-#[derive(Debug)]
-pub enum Function {
-    FilePath(PathBuf),
-    FileRegex(Regex),
-    ActivePath(PathBuf),
-    ActiveRegex(Regex),
-    Many(Regex),
-    ManyActive(Regex),
-    Checksum(PathBuf, u32),
-    Version(PathBuf, String, ComparisonOperator),
-}
-
 const INVALID_PATH_CHARS: &str = "\":*?<>|\\"; // \ is treated as invalid to distinguish regex strings.
 const INVALID_REGEX_PATH_CHARS: &str = "\"<>";
 
@@ -92,15 +70,6 @@ fn parse_checksum_args(input: &str) -> IResult<&str, (PathBuf, u32)> {
 }
 
 impl Function {
-    pub fn eval(&self) -> Result<bool, Error> {
-        // TODO: Handle all variants.
-        // TODO: Paths may not lead outside game directory.
-        match *self {
-            Function::FilePath(ref f) => Ok(f.exists()),
-            _ => Ok(false),
-        }
-    }
-
     pub fn parse(input: &str) -> IResult<&str, Function> {
         do_parse!(
             input,
@@ -241,68 +210,4 @@ mod tests {
             _ => panic!("Expected a checksum function"),
         }
     }
-
-    #[test]
-    fn function_file_path_eval_should_return_true_if_the_file_exists_relative_to_the_data_path() {
-        let function = Function::FilePath(PathBuf::from("Cargo.toml"));
-
-        assert!(function.eval().unwrap());
-
-        unimplemented!("not yet any way to actually specify the data path");
-    }
-
-    #[test]
-    fn function_file_path_eval_should_return_true_if_given_a_plugin_that_is_ghosted() {
-        let function = Function::FilePath(PathBuf::from("test.esp"));
-
-        assert!(function.eval().unwrap());
-
-        unimplemented!("need to add tempdir and create a test.esp.ghost");
-    }
-
-    #[test]
-    fn function_file_path_eval_should_be_true_if_given_LOOT() {
-        unimplemented!();
-    }
-
-    #[test]
-    fn function_file_path_eval_should_not_check_for_ghosted_non_plugin_file() {
-        unimplemented!();
-    }
-
-    #[test]
-    fn function_file_path_eval_should_error_if_the_path_is_outside_game_directory() {
-        unimplemented!("to do");
-    }
-
-    #[test]
-    fn function_file_path_eval_should_return_false_if_the_file_does_not_exist() {
-        let function = Function::FilePath(PathBuf::from("missing"));
-
-        assert!(!function.eval().unwrap());
-    }
-
-    #[test]
-    fn function_file_regex_eval_should_error_if_the_path_is_outside_game_directory() {
-        unimplemented!();
-    }
-
-    #[test]
-    fn function_file_regex_eval_should_be_false_if_no_directory_entries_match() {
-        unimplemented!();
-    }
-
-    #[test]
-    fn function_file_regex_eval_should_be_false_if_the_parent_path_part_is_not_a_directory() {
-        unimplemented!();
-    }
-
-    #[test]
-    fn function_file_regex_eval_should_be_true_if_a_directory_entry_matches() {
-        unimplemented!();
-    }
-
-    #[test]
-    fn function_active_path_eval_should_be_false_if
-
 }