diff --git a/CHANGELOG b/CHANGELOG
new file mode 100644
index 0000000..a5b4bac
--- /dev/null
+++ b/CHANGELOG
@@ -0,0 +1,9 @@
+0.5.1
+-----
+Change to how errors are exported
+Add a few new callable functions
+
+0.5.0
+-----
+Massive overhaul to api
+Enhance documentation
\ No newline at end of file
diff --git a/src/functions/str.rs b/src/functions/str.rs
new file mode 100644
index 0000000..54d7c9a
--- /dev/null
+++ b/src/functions/str.rs
@@ -0,0 +1,148 @@
+use crate::value::{Value, IntegerType};
+use crate::errors::*;
+
+pub fn builtin_contains(args: &[Value]) -> Result<Value, ParserError> {
+    if args.len() != 2 {
+        return Err(ParserError::FunctionNArg(FunctionNArgError::new("contains(source, s)", 1, 1)));
+    }
+
+    Ok(Value::Boolean(args[0].as_string().contains(&args[1].as_string())))
+}
+
+pub fn builtin_strlen(args: &[Value]) -> Result<Value, ParserError> {
+    if args.len() != 1 {
+        return Err(ParserError::FunctionNArg(FunctionNArgError::new("strlen(s)", 1, 1)));
+    }
+
+    match &args[0] {
+        Value::String(s) => Ok(Value::Integer(s.len() as IntegerType)),
+        _ => Err(ParserError::FunctionArgType(FunctionArgTypeError::new("strlen(s)", 1, ExpectedTypes::String)))
+    }
+}
+
+pub fn builtin_concat(args: &[Value]) -> Result<Value, ParserError> {
+    if args.is_empty() {
+        return Err(ParserError::FunctionNArg(FunctionNArgError::new("concat(s, s2, ...)", 1, 1)));
+    }
+
+    Ok(Value::String(args.iter().map(|v|v.as_string()).collect::<String>()))
+}
+
+pub fn builtin_uppercase(args: &[Value]) -> Result<Value, ParserError> {
+    if args.is_empty() {
+        return Err(ParserError::FunctionNArg(FunctionNArgError::new("uppercase(s)", 1, 1)));
+    }
+
+    Ok(Value::String(args[0].as_string().to_uppercase()))
+}
+
+pub fn builtin_lowercase(args: &[Value]) -> Result<Value, ParserError> {
+    if args.is_empty() {
+        return Err(ParserError::FunctionNArg(FunctionNArgError::new("lowercase(s)", 1, 1)));
+    }
+
+    Ok(Value::String(args[0].as_string().to_lowercase()))
+}
+
+pub fn builtin_trim(args: &[Value]) -> Result<Value, ParserError> {
+    if args.is_empty() {
+        return Err(ParserError::FunctionNArg(FunctionNArgError::new("trim(s)", 1, 1)));
+    }
+
+    Ok(Value::String(args[0].as_string().trim().to_string()))
+}
+
+pub fn builtin_substr(args: &[Value]) -> Result<Value, ParserError> {
+    if args.len() != 2 && args.len() != 3 {
+        return Err(ParserError::FunctionNArg(FunctionNArgError::new("substr(s, start, [length])", 2, 3)));
+    }
+
+    let start = match args[1].as_int() {
+        Some(n) => n,
+        None => return Err(ParserError::FunctionArgType(FunctionArgTypeError::new("substr(s, start, [length])", 2, ExpectedTypes::IntOrFloat)))
+    };
+
+    match &args[0] {
+        Value::String(s) => {
+            let length = if args.len() == 3 { match args[2].as_int() {
+                Some(n) => n,
+                None => return Err(ParserError::FunctionArgType(FunctionArgTypeError::new("substr(s, start, [length])", 3, ExpectedTypes::IntOrFloat)))
+            } } else { s.len() as IntegerType - start };
+            if start >= s.len() as IntegerType || start < 0 {
+                return Err(ParserError::FunctionArgOverFlow(FunctionArgOverFlowError::new("substr(s, start, [length])", 2)));
+            } else if length < 0 || length > (s.len() - start as usize) as IntegerType {
+                return Err(ParserError::FunctionArgOverFlow(FunctionArgOverFlowError::new("substr(s, start, [length])", 3)));
+            }
+
+            Ok(Value::String(s.chars().skip(start as usize).take(length as usize).collect()))
+        },
+        _ => Err(ParserError::FunctionArgType(FunctionArgTypeError::new("substr(s, start, [length])", 1, ExpectedTypes::String)))
+    }
+}
+
+#[cfg(test)]
+mod test_builtin_functions {
+    use super::*;
+
+    #[test]
+    fn test_strlen() {
+        assert_eq!(Value::Integer(0), builtin_strlen(&[Value::String("".to_string())]).unwrap());
+        assert_eq!(Value::Integer(3), builtin_strlen(&[Value::String("   ".to_string())]).unwrap());
+    }
+
+    #[test]
+    fn test_uppercase() {
+        assert_eq!(Value::String("TEST".to_string()), builtin_uppercase(&[Value::String("test".to_string())]).unwrap());
+        assert_eq!(Value::String(" TEST  ".to_string()), builtin_uppercase(&[Value::String(" test  ".to_string())]).unwrap());
+    }
+
+
+    #[test]
+    fn test_lowercase() {
+        assert_eq!(Value::String("test".to_string()), builtin_lowercase(&[Value::String("TEST".to_string())]).unwrap());
+        assert_eq!(Value::String(" test  ".to_string()), builtin_lowercase(&[Value::String(" TEST  ".to_string())]).unwrap());
+    }
+
+
+    #[test]
+    fn test_trim() {
+        assert_eq!(Value::String("test".to_string()), builtin_trim(&[Value::String("test".to_string())]).unwrap());
+        assert_eq!(Value::String("TEST".to_string()), builtin_trim(&[Value::String(" TEST  ".to_string())]).unwrap());
+    }
+
+    #[test]
+    fn test_concat() {
+        assert_eq!(Value::String(" ".to_string()), builtin_concat(
+            &[Value::String("".to_string()), 
+            Value::String(" ".to_string())
+        ]).unwrap());
+        assert_eq!(Value::String("test4false".to_string()), builtin_concat(
+            &[Value::String("test".to_string()), 
+            Value::Integer(4),
+            Value::Boolean(false)
+        ]).unwrap());
+    }
+    
+    #[test]
+    fn test_substr() {
+        assert_eq!(Value::String("t".to_string()), builtin_substr(
+            &[Value::String("test".to_string()), Value::Integer(3)]
+            
+        ).unwrap());
+        assert_eq!(Value::String("tes".to_string()), builtin_substr(
+            &[Value::String("test".to_string()), Value::Integer(0), Value::Integer(3)], 
+        ).unwrap());
+    }
+    
+    #[test]
+    fn test_contains() {
+        assert_eq!(Value::Boolean(true), builtin_contains(
+            &[Value::String("test".to_string()), Value::String("e".to_string())]
+            
+        ).unwrap());
+        assert_eq!(Value::Boolean(false), builtin_contains(
+            &[Value::String("test".to_string()), Value::String("fff".to_string())]
+            
+        ).unwrap());
+    }
+}
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