diff --git a/mers_lib/src/program/configs/with_base.rs b/mers_lib/src/program/configs/with_base.rs
index 1afd53a..a3b7fbb 100755
--- a/mers_lib/src/program/configs/with_base.rs
+++ b/mers_lib/src/program/configs/with_base.rs
@@ -11,13 +11,37 @@ impl Config {
     /// `deref: fn` clones the value from a reference
     /// `eq: fn` returns true if all the values are equal, otherwise false.
     /// `loop: fn` runs a function until it returns (T) instead of (), then returns T.
+    /// `len: fn` gets the length of strings or tuples
     pub fn with_base(self) -> Self {
         self.add_var(
+            "len".to_string(),
+            Data::new(data::function::Function {
+                info: Arc::new(Info::neverused()),
+                info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
+                out: Arc::new(|a, _i| {
+                    for t in &a.types {
+                        if t.as_any().downcast_ref::<data::string::StringT>().is_none() && t.as_any().downcast_ref::<data::tuple::TupleT>().is_none() {
+                            return Err(crate::program::run::CheckError(format!("cannot get length of {t} (must be a tuple or a string)")));
+                        }
+                    }
+                    Ok(Type::new(data::int::IntT))
+                }),
+                run: Arc::new(|a, _i| {
+                    if let Some(t) = a.get().as_any().downcast_ref::<data::tuple::Tuple>() {
+                        Data::new(data::int::Int(t.0.len() as _))
+                    } else if let Some(s) = a.get().as_any().downcast_ref::<data::string::String>() {
+                        Data::new(data::int::Int(s.0.len() as _))
+                    } else {
+                        unreachable!("called len on {a:?}, which isn't a tuple or a string")
+                    }
+                }),
+            }),
+        ).add_var(
             "loop".to_string(),
             Data::new(data::function::Function {
                 info: Arc::new(Info::neverused()),
                 info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
-                out: Arc::new(|a, i| {
+                out: Arc::new(|a, _i| {
                     let mut o = Type::empty();
                     for t in &a.types {
                         if let Some(t) = t.as_any().downcast_ref::<data::function::FunctionT>() {
@@ -56,7 +80,7 @@ impl Config {
             Data::new(data::function::Function {
                 info: Arc::new(Info::neverused()),
                 info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
-                out: Arc::new(|a, i| {
+                out: Arc::new(|a, _i| {
                     for t in &a.types {
                             if t.iterable().is_none() {
                                 return Err(crate::program::run::CheckError(format!("called eq on non-iterable")))
@@ -89,7 +113,7 @@ impl Config {
             Data::new(data::function::Function {
                 info: Arc::new(Info::neverused()),
                 info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
-                out: Arc::new(|a, i| if let Some(v) = a.dereference() { Ok(v) } else { Err(crate::program::run::CheckError(format!("cannot dereference type {a}")))}),
+                out: Arc::new(|a, _i| if let Some(v) = a.dereference() { Ok(v) } else { Err(crate::program::run::CheckError(format!("cannot dereference type {a}")))}),
                 run: Arc::new(|a, _i| {
                     if let Some(r) = a
                         .get()