diff --git a/src/tools/mod.rs b/src/tools/mod.rs
index 9f097c7..bc99c29 100644
--- a/src/tools/mod.rs
+++ b/src/tools/mod.rs
@@ -1,4 +1,9 @@
-//! This module contains dust's built-in commands.
+//! Dust's built-in commands.
+//!
+//! When a tool in invoked in Dust, the input is checked against the inputs listed in its ToolInfo.
+//! The input should then be double-checked by `Tool::check_input` when you implement `run`. The
+//! purpose of the second check is to weed out mistakes in how the inputs were described in the
+//! ToolInfo. The errors from the second check should only come up during development and should not //! be seen by the user.
 //!
 //! ## Writing macros
 //!
@@ -43,10 +48,9 @@ pub mod random;
 pub mod system;
 pub mod time;
 
-/// Master list of all macros.
+/// Master list of all tools.
 ///
-/// This list is used to match identifiers with macros and to provide info to
-/// the shell.
+/// This list is used to match identifiers with tools and to provide info to the shell.
 pub const TOOL_LIST: [&'static dyn Tool; 57] = [
     &collections::Count,
     &collections::CreateTable,
@@ -111,6 +115,29 @@ pub const TOOL_LIST: [&'static dyn Tool; 57] = [
 pub trait Tool: Sync + Send {
     fn info(&self) -> ToolInfo<'static>;
     fn run(&self, argument: &Value) -> Result<Value>;
+
+    fn check_type<'a>(&self, argument: &'a Value) -> Result<&'a Value> {
+        if self
+            .info()
+            .inputs
+            .iter()
+            .any(|value_type| &argument.value_type() == value_type)
+        {
+            Ok(argument)
+        } else {
+            Err(crate::Error::TypeCheckFailure {
+                tool_info: self.info(),
+                argument: argument.clone(),
+            })
+        }
+    }
+
+    fn fail(&self, argument: &Value) -> Result<Value> {
+        Err(crate::Error::TypeCheckFailure {
+            tool_info: self.info(),
+            argument: argument.clone(),
+        })
+    }
 }
 
 /// Information needed for each macro.
diff --git a/src/tools/random.rs b/src/tools/random.rs
index a4e14a7..a9e93da 100644
--- a/src/tools/random.rs
+++ b/src/tools/random.rs
@@ -17,24 +17,14 @@ impl Tool for RandomBoolean {
     }
 
     fn run(&self, argument: &Value) -> Result<Value> {
-        match argument {
-            Value::Empty => {
-                let boolean = rand::thread_rng().gen();
+        let argument = self.check_type(argument)?;
 
-                Ok(Value::Boolean(boolean))
-            }
-            Value::String(_)
-            | Value::Float(_)
-            | Value::Integer(_)
-            | Value::Boolean(_)
-            | Value::List(_)
-            | Value::Map(_)
-            | Value::Table(_)
-            | Value::Time(_)
-            | Value::Function(_) => Err(Error::TypeCheckFailure {
-                tool_info: self.info(),
-                argument: argument.clone(),
-            }),
+        if let Value::Empty = argument {
+            let boolean = rand::thread_rng().gen();
+
+            Ok(Value::Boolean(boolean))
+        } else {
+            self.fail(argument)
         }
     }
 }
@@ -72,10 +62,7 @@ impl Tool for RandomInteger {
                 Ok(Value::Integer(integer))
             }
             Value::Empty => Ok(crate::Value::Integer(random())),
-            _ => Err(Error::TypeCheckFailure {
-                tool_info: self.info(),
-                argument: argument.clone(),
-            }),
+            _ => self.fail(argument),
         }
     }
 }
@@ -93,35 +80,34 @@ impl Tool for RandomString {
     }
 
     fn run(&self, argument: &Value) -> Result<Value> {
-        match argument {
-            Value::Integer(length) => {
-                let length: usize = length.unsigned_abs().try_into().unwrap_or(0);
-                let mut random = String::with_capacity(length);
+        let argument = self.check_type(argument)?;
 
-                for _ in 0..length {
-                    let random_char = thread_rng().gen_range('A'..='z').to_string();
+        if let Value::Integer(length) = argument {
+            let length: usize = length.unsigned_abs().try_into().unwrap_or(0);
+            let mut random = String::with_capacity(length);
 
-                    random.push_str(&random_char);
-                }
+            for _ in 0..length {
+                let random_char = thread_rng().gen_range('A'..='z').to_string();
 
-                Ok(Value::String(random))
+                random.push_str(&random_char);
             }
-            Value::Empty => {
-                let mut random = String::with_capacity(10);
 
-                for _ in 0..10 {
-                    let random_char = thread_rng().gen_range('A'..='z').to_string();
-
-                    random.push_str(&random_char);
-                }
-
-                Ok(Value::String(random))
-            }
-            _ => Err(Error::TypeCheckFailure {
-                tool_info: self.info(),
-                argument: argument.clone(),
-            }),
+            return Ok(Value::String(random));
         }
+
+        if let Value::Empty = argument {
+            let mut random = String::with_capacity(10);
+
+            for _ in 0..10 {
+                let random_char = thread_rng().gen_range('A'..='z').to_string();
+
+                random.push_str(&random_char);
+            }
+
+            return Ok(Value::String(random));
+        }
+
+        self.fail(argument)
     }
 }
 
@@ -133,14 +119,18 @@ impl Tool for RandomFloat {
             identifier: "random_float",
             description: "Generate a random floating point value between 0 and 1.",
             group: "random",
-            inputs: vec![],
+            inputs: vec![ValueType::Empty],
         }
     }
 
     fn run(&self, argument: &Value) -> Result<Value> {
-        argument.as_empty()?;
+        let argument = self.check_type(argument)?;
 
-        Ok(Value::Float(random()))
+        if argument.is_empty() {
+            Ok(Value::Float(random()))
+        } else {
+            self.fail(argument)
+        }
     }
 }
 
@@ -150,22 +140,22 @@ impl Tool for Random {
     fn info(&self) -> ToolInfo<'static> {
         ToolInfo {
             identifier: "random",
-            description: "Select a random item from a collection.",
+            description: "Select a random item from a list.",
             group: "random",
-            inputs: vec![],
+            inputs: vec![ValueType::List],
         }
     }
 
     fn run(&self, argument: &Value) -> Result<Value> {
-        if let Ok(list) = argument.as_list() {
+        let argument = self.check_type(argument)?;
+
+        if let Value::List(list) = argument {
             let random_index = thread_rng().gen_range(0..list.len());
             let random_item = list.get(random_index).unwrap();
 
             Ok(random_item.clone())
         } else {
-            Err(Error::ExpectedCollection {
-                actual: argument.clone(),
-            })
+            self.fail(argument)
         }
     }
 }