diff --git a/add_integers.uplc b/add_integers.uplc
index 2d2d3aa9..eab073b4 100644
--- a/add_integers.uplc
+++ b/add_integers.uplc
@@ -1,4 +1,7 @@
 (program
   1.0.0
-  [ (builtin ifThenElse) (con bool True) (con integer 1) (con string "yo") ]
-)
+  [
+    [ [ (force (builtin ifThenElse)) (con integer 2) ] (con integer 1) ]
+    (con string "yo")
+  ]
+)
\ No newline at end of file
diff --git a/crates/cli/src/args.rs b/crates/cli/src/args.rs
index 02a850ec..fcbe306c 100644
--- a/crates/cli/src/args.rs
+++ b/crates/cli/src/args.rs
@@ -32,8 +32,11 @@ pub enum UplcCommand {
         out: Option<String>,
     },
     /// Format an Untyped Plutus Core program
-    Fmt { input: PathBuf },
-
+    Fmt {
+        input: PathBuf,
+        #[clap(short, long)]
+        print: bool,
+    },
     /// Evaluate an Untyped Plutus Core program
     Eval {
         input: PathBuf,
diff --git a/crates/cli/src/main.rs b/crates/cli/src/main.rs
index 2756d2bd..bd2f2880 100644
--- a/crates/cli/src/main.rs
+++ b/crates/cli/src/main.rs
@@ -1,4 +1,4 @@
-use std::fs;
+use std::{fmt::Write as _, fs};
 
 use uplc::{
     ast::{DeBruijn, FakeNamedDeBruijn, Name, NamedDeBruijn, Program, Term},
@@ -27,7 +27,7 @@ fn main() -> anyhow::Result<()> {
                     let mut output = String::new();
 
                     for (i, byte) in bytes.iter().enumerate() {
-                        output.push_str(&format!("{:08b}", byte));
+                        let _ = write!(output, "{:08b}", byte);
 
                         if (i + 1) % 4 == 0 {
                             output.push('\n');
@@ -47,14 +47,18 @@ fn main() -> anyhow::Result<()> {
                     fs::write(&out_name, &bytes)?;
                 }
             }
-            UplcCommand::Fmt { input } => {
+            UplcCommand::Fmt { input, print } => {
                 let code = std::fs::read_to_string(&input)?;
 
                 let program = parser::program(&code)?;
 
                 let pretty = program.to_pretty();
 
-                fs::write(&input, pretty)?;
+                if print {
+                    println!("{}", pretty);
+                } else {
+                    fs::write(&input, pretty)?;
+                }
             }
             UplcCommand::Unflat { input, print, out } => {
                 let bytes = std::fs::read(&input)?;
@@ -105,7 +109,7 @@ fn main() -> anyhow::Result<()> {
                     }
                 }
 
-                println!("Costs - memory: {} & cpu: {}", cost.mem, cost.cpu);
+                println!("\nCosts - memory: {} & cpu: {}", cost.mem, cost.cpu);
             }
         },
     }
diff --git a/crates/uplc/src/ast.rs b/crates/uplc/src/ast.rs
index 58c20adb..2443de28 100644
--- a/crates/uplc/src/ast.rs
+++ b/crates/uplc/src/ast.rs
@@ -162,7 +162,7 @@ impl PartialEq for NamedDeBruijn {
 /// It allows for injecting fake textual names while also using Debruijn for decoding
 /// without having to loop through twice.
 #[derive(Debug, Clone, PartialEq)]
-pub struct FakeNamedDeBruijn(NamedDeBruijn);
+pub struct FakeNamedDeBruijn(pub NamedDeBruijn);
 
 impl From<DeBruijn> for FakeNamedDeBruijn {
     fn from(d: DeBruijn) -> Self {
diff --git a/crates/uplc/src/flat.rs b/crates/uplc/src/flat.rs
index 23eb3abe..3e4663ec 100644
--- a/crates/uplc/src/flat.rs
+++ b/crates/uplc/src/flat.rs
@@ -18,6 +18,7 @@ const TERM_TAG_WIDTH: u32 = 4;
 pub trait Binder<'b>: Encode + Decode<'b> {
     fn binder_encode(&self, e: &mut Encoder) -> Result<(), en::Error>;
     fn binder_decode(d: &mut Decoder) -> Result<Self, de::Error>;
+    fn text(&self) -> &str;
 }
 
 impl<'b, T> Flat<'b> for Program<T> where T: Binder<'b> + Debug {}
@@ -246,6 +247,10 @@ impl<'b> Binder<'b> for Name {
     fn binder_decode(d: &mut Decoder) -> Result<Self, de::Error> {
         Name::decode(d)
     }
+
+    fn text(&self) -> &str {
+        &self.text
+    }
 }
 
 impl Encode for NamedDeBruijn {
@@ -279,6 +284,10 @@ impl<'b> Binder<'b> for NamedDeBruijn {
             index: DeBruijn::new(0),
         })
     }
+
+    fn text(&self) -> &str {
+        &self.text
+    }
 }
 
 impl Encode for DeBruijn {
@@ -303,6 +312,10 @@ impl<'b> Binder<'b> for DeBruijn {
     fn binder_decode(_d: &mut Decoder) -> Result<Self, de::Error> {
         Ok(DeBruijn::new(0))
     }
+
+    fn text(&self) -> &str {
+        "i"
+    }
 }
 
 impl Encode for FakeNamedDeBruijn {
@@ -333,6 +346,10 @@ impl<'b> Binder<'b> for FakeNamedDeBruijn {
 
         Ok(index.into())
     }
+
+    fn text(&self) -> &str {
+        &self.0.text
+    }
 }
 
 impl Encode for DefaultFunction {
diff --git a/crates/uplc/src/machine.rs b/crates/uplc/src/machine.rs
index 4a262b8b..540dfd39 100644
--- a/crates/uplc/src/machine.rs
+++ b/crates/uplc/src/machine.rs
@@ -10,7 +10,7 @@ mod runtime;
 use cost_model::{ExBudget, StepKind};
 pub use error::Error;
 
-use self::{cost_model::CostModel, runtime::BuiltinRuntime};
+use self::{cost_model::CostModel, error::Type, runtime::BuiltinRuntime};
 
 pub struct Machine {
     costs: CostModel,
@@ -198,7 +198,7 @@ impl Machine {
                 term,
                 mut runtime,
             } => {
-                let force_term = Term::Force(Box::new(dbg!(term)));
+                let force_term = Term::Force(Box::new(term));
 
                 if runtime.needs_force() {
                     runtime.consume_force();
@@ -207,7 +207,7 @@ impl Machine {
 
                     self.return_compute(res)
                 } else {
-                    todo!()
+                    Err(Error::BuiltinTermArgumentExpected(force_term))
                 }
             }
             rest => Err(Error::NonPolymorphicInstantiation(rest)),
@@ -245,7 +245,7 @@ impl Machine {
 
                     self.return_compute(res)
                 } else {
-                    todo!()
+                    Err(Error::UnexpectedBuiltinTermArgument(t))
                 }
             }
             rest => Err(Error::NonFunctionalApplication(rest)),
@@ -367,7 +367,7 @@ impl Value {
                     } else {
                         //TODO
                         //    std::mem::size_of( i.abs()
-                        todo!()
+                        1
                     }
                 }
                 Constant::ByteString(b) => (((b.len() - 1) / 8) + 1) as i64,
@@ -382,3 +382,21 @@ impl Value {
         }
     }
 }
+
+impl From<&Value> for Type {
+    fn from(value: &Value) -> Self {
+        match value {
+            Value::Con(constant) => match constant {
+                Constant::Integer(_) => Type::Integer,
+                Constant::ByteString(_) => Type::ByteString,
+                Constant::String(_) => Type::String,
+                Constant::Char(_) => todo!(),
+                Constant::Unit => Type::Unit,
+                Constant::Bool(_) => Type::Bool,
+            },
+            Value::Delay(_) => todo!(),
+            Value::Lambda { .. } => todo!(),
+            Value::Builtin { .. } => todo!(),
+        }
+    }
+}
diff --git a/crates/uplc/src/machine/error.rs b/crates/uplc/src/machine/error.rs
index 25a30175..b104906d 100644
--- a/crates/uplc/src/machine/error.rs
+++ b/crates/uplc/src/machine/error.rs
@@ -1,3 +1,5 @@
+use std::fmt::Display;
+
 use thiserror::Error;
 
 use crate::ast::{NamedDeBruijn, Term};
@@ -18,4 +20,31 @@ pub enum Error {
     NonPolymorphicInstantiation(Value),
     #[error("Attempted to apply a non-function: {0:#?}")]
     NonFunctionalApplication(Value),
+    #[error("Type mismatch expected '{0}' got '{1}'")]
+    TypeMismatch(Type, Type),
+    #[error("A builtin received a term argument when something else was expected:\n\n{}\n\nYou probably forgot to wrap the builtin with a force.", .0.to_pretty())]
+    UnexpectedBuiltinTermArgument(Term<NamedDeBruijn>),
+    #[error("A builtin expected a term argument, but something else was received:\n\n{}\n\nYou probably have an extra force wrapped around a builtin", .0.to_pretty())]
+    BuiltinTermArgumentExpected(Term<NamedDeBruijn>),
+}
+
+#[derive(Debug, Clone)]
+pub enum Type {
+    Bool,
+    Integer,
+    String,
+    ByteString,
+    Unit,
+}
+
+impl Display for Type {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Type::Bool => write!(f, "bool"),
+            Type::Integer => write!(f, "integer"),
+            Type::String => write!(f, "string"),
+            Type::ByteString => write!(f, "bytestring"),
+            Type::Unit => write!(f, "unit"),
+        }
+    }
 }
diff --git a/crates/uplc/src/machine/runtime.rs b/crates/uplc/src/machine/runtime.rs
index e3d0d779..a213e320 100644
--- a/crates/uplc/src/machine/runtime.rs
+++ b/crates/uplc/src/machine/runtime.rs
@@ -2,9 +2,8 @@ use crate::{ast::Constant, builtins::DefaultFunction};
 
 use super::{
     cost_model::{BuiltinCosts, ExBudget},
-    // cost_model::{CostingFun, ExBudget},
-    Error,
-    Value,
+    error::Type,
+    Error, Value,
 };
 
 //#[derive(std::cmp::PartialEq)]
@@ -13,12 +12,6 @@ use super::{
 //    Deferred,
 //}
 
-// pub struct BuiltinRuntimeOptions<T, G> {
-//     immediate_eval: T,
-//     deferred_eval: T,
-//     budget: fn(CostingFun<G>) -> fn(Vec<u32>) -> ExBudget,
-// }
-
 #[derive(Clone, Debug)]
 pub struct BuiltinRuntime {
     args: Vec<Value>,
@@ -35,16 +28,6 @@ impl BuiltinRuntime {
         }
     }
 
-    // fn from_builtin_runtime_options<G>(
-    //     eval_mode: EvalMode,
-    //     cost: CostingFun<G>,
-    //     runtime_options: BuiltinRuntimeOptions<T, G>,
-    // ) -> BuiltinRuntime {
-    //    Self {
-    //      budget: (runtime_options.budget)(cost),
-    //    }
-    // }
-
     pub fn is_arrow(&self) -> bool {
         self.args.len() != self.fun.arity()
     }
@@ -209,7 +192,7 @@ impl DefaultFunction {
                 if arg.is_integer() {
                     Ok(())
                 } else {
-                    todo!("type error")
+                    Err(Error::TypeMismatch(Type::Integer, arg.into()))
                 }
             }
             DefaultFunction::SubtractInteger => todo!(),
@@ -240,15 +223,11 @@ impl DefaultFunction {
             DefaultFunction::EncodeUtf8 => todo!(),
             DefaultFunction::DecodeUtf8 => todo!(),
             DefaultFunction::IfThenElse => {
-                if args.is_empty() {
-                    if arg.is_bool() {
-                        return Ok(());
-                    } else {
-                        todo!("type error")
-                    }
+                if args.is_empty() && !arg.is_bool() {
+                    Err(Error::TypeMismatch(Type::Bool, arg.into()))
+                } else {
+                    Ok(())
                 }
-
-                Ok(())
             }
             DefaultFunction::ChooseUnit => todo!(),
             DefaultFunction::Trace => todo!(),
@@ -278,18 +257,19 @@ impl DefaultFunction {
         }
     }
 
+    // This should be safe because we've already checked
+    // the types of the args as they were pushed. Although
+    // the unreachables look ugly, it's the reality of the situation.
     pub fn call(&self, args: &[Value]) -> Result<Value, Error> {
         match self {
             DefaultFunction::AddInteger => {
-                assert_eq!(args.len(), self.arity());
-
                 let args = (&args[0], &args[1]);
 
                 match args {
                     (Value::Con(Constant::Integer(arg1)), Value::Con(Constant::Integer(arg2))) => {
                         Ok(Value::Con(Constant::Integer(arg1 + arg2)))
                     }
-                    _ => todo!("handle error"),
+                    _ => unreachable!(),
                 }
             }
             DefaultFunction::SubtractInteger => todo!(),
@@ -327,7 +307,7 @@ impl DefaultFunction {
                         Ok(args[2].clone())
                     }
                 }
-                _ => todo!("handle error"),
+                _ => unreachable!(),
             },
             DefaultFunction::ChooseUnit => todo!(),
             DefaultFunction::Trace => todo!(),
diff --git a/crates/uplc/src/parser.rs b/crates/uplc/src/parser.rs
index 2620ea1c..fc940c8f 100644
--- a/crates/uplc/src/parser.rs
+++ b/crates/uplc/src/parser.rs
@@ -82,10 +82,10 @@ peg::parser! {
           }
 
         rule delay() -> Term<Name>
-          = "(" _* "delay" _+ t:term() _* ")" { Term::Delay(Box::new(t)) }
+          = "(" _* "delay" _* t:term() _* ")" { Term::Delay(Box::new(t)) }
 
         rule force() -> Term<Name>
-          = "(" _* "force" _+ t:term() _* ")" { Term::Force(Box::new(t)) }
+          = "(" _* "force" _* t:term() _* ")" { Term::Force(Box::new(t)) }
 
         rule error() -> Term<Name>
           = "(" _* "error" _* ")" { Term::Error }
diff --git a/crates/uplc/src/pretty.rs b/crates/uplc/src/pretty.rs
index d2e72003..23bdbe1c 100644
--- a/crates/uplc/src/pretty.rs
+++ b/crates/uplc/src/pretty.rs
@@ -1,8 +1,14 @@
 use pretty::RcDoc;
 
-use crate::ast::{Constant, Name, Program, Term};
+use crate::{
+    ast::{Constant, Program, Term},
+    flat::Binder,
+};
 
-impl Program<Name> {
+impl<'a, T> Program<T>
+where
+    T: Binder<'a>,
+{
     pub fn to_pretty(&self) -> String {
         let mut w = Vec::new();
 
@@ -40,7 +46,10 @@ impl Program<Name> {
     }
 }
 
-impl Term<Name> {
+impl<'a, T> Term<T>
+where
+    T: Binder<'a>,
+{
     pub fn to_pretty(&self) -> String {
         let mut w = Vec::new();
 
@@ -65,7 +74,7 @@ impl Term<Name> {
 
     fn to_doc(&self) -> RcDoc<()> {
         match self {
-            Term::Var(name) => RcDoc::text(&name.text),
+            Term::Var(name) => RcDoc::text(name.text()),
             Term::Delay(term) => RcDoc::text("(")
                 .append(
                     RcDoc::text("delay")
@@ -82,7 +91,7 @@ impl Term<Name> {
                 .append(
                     RcDoc::text("lam")
                         .append(RcDoc::line())
-                        .append(RcDoc::text(&parameter_name.text))
+                        .append(RcDoc::text(parameter_name.text()))
                         .append(RcDoc::line())
                         .append(body.to_doc())
                         .nest(2),
@@ -161,7 +170,7 @@ impl Constant {
                 .append(RcDoc::text("()")),
             Constant::Bool(b) => RcDoc::text("bool")
                 .append(RcDoc::line())
-                .append(RcDoc::text(if *b { "true" } else { "false" })),
+                .append(RcDoc::text(if *b { "True" } else { "False" })),
         }
     }
 }