diff --git a/sample/cp.pl.xml b/sample/cp.pl.xml
new file mode 100644
index 0000000..f941bb7
--- /dev/null
+++ b/sample/cp.pl.xml
@@ -0,0 +1,30 @@
+<program name="files">
+    <main>
+        <assign variable="args">
+            <call function="get-args">
+                <arguments />
+            </call>
+        </assign>
+        <call function="write-file">
+            <arguments>
+                <call function="array-get">
+                    <arguments>
+                        <value variable="args" />
+                        <integer value="2" />
+                    </arguments>
+                </call>
+                <call function="read-file">
+                    <arguments>
+                        <call function="array-get">
+                            <arguments>
+                                <value variable="args" />
+                                <integer value="1" />
+                            </arguments>
+                        </call>
+                    </arguments>
+                </call>
+                <integer value="0" />
+            </arguments>
+        </call>
+    </main>
+</program>
diff --git a/src/error.rs b/src/error.rs
index b57f360..78e0293 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -24,11 +24,15 @@ impl fmt::Display for MissingAttribute {
 impl Error for MissingAttribute {}
 
 #[derive(Clone, Debug)]
-pub struct BadArgumentCount(pub &'static str, pub usize);
+pub struct BadArgumentCount(pub &'static str, pub usize, pub usize);
 
 impl fmt::Display for BadArgumentCount {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "bad argument count ({}) in call to '{}'", self.1, self.0)
+        write!(
+            f,
+            "bad argument count ({}, expected {}) in call to '{}'",
+            self.1, self.2, self.0
+        )
     }
 }
 
diff --git a/src/stl.rs b/src/stl.rs
index 96389e5..8396105 100644
--- a/src/stl.rs
+++ b/src/stl.rs
@@ -2,11 +2,13 @@ use super::error::{BadArgumentCount, InvalidArgument};
 use super::{Context, Value};
 use std::cell::RefCell;
 use std::error::Error;
-use std::io;
+use std::fs::{self, OpenOptions};
+use std::io::{stdin, stdout, Write};
 use std::rc::Rc;
 
 pub fn inject_all(ctx: &mut Context) {
     ctx.assign(String::from("print"), Value::StdFunction(print));
+    ctx.assign(String::from("print-line"), Value::StdFunction(print_line));
     ctx.assign(String::from("input"), Value::StdFunction(input));
     ctx.assign(
         String::from("string-split"),
@@ -23,9 +25,26 @@ pub fn inject_all(ctx: &mut Context) {
     ctx.assign(String::from("to-ascii"), Value::StdFunction(to_ascii));
     ctx.assign(String::from("from-ascii"), Value::StdFunction(from_ascii));
     ctx.assign(String::from("get-args"), Value::StdFunction(get_args));
+    ctx.assign(String::from("write-file"), Value::StdFunction(write_file));
+    ctx.assign(String::from("read-file"), Value::StdFunction(read_file));
 }
 
 fn print(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
+    if vals.len() == 1 {
+        match &vals[0] {
+            Value::Integer(i) => print!("{}", i),
+            Value::Float(f) => print!("{}", f),
+            Value::String(s) => print!("{}", s),
+            v => print!("{:?}", v), // _ => Err("unprintable value")?,
+        };
+        let _ = stdout().flush();
+        Ok(Some(vals[0].clone()))
+    } else {
+        Err(BadArgumentCount("print", vals.len(), 1).into())
+    }
+}
+
+fn print_line(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
     if vals.len() == 1 {
         match &vals[0] {
             Value::Integer(i) => println!("{}", i),
@@ -33,20 +52,21 @@ fn print(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
             Value::String(s) => println!("{}", s),
             v => println!("{:?}", v), // _ => Err("unprintable value")?,
         };
+        let _ = stdout().flush();
         Ok(Some(vals[0].clone()))
     } else {
-        Err(BadArgumentCount("print", vals.len()).into())
+        Err(BadArgumentCount("print-line", vals.len(), 1).into())
     }
 }
 
 fn input(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
     if vals.len() == 0 {
         let mut line = String::new();
-        io::stdin().read_line(&mut line)?;
+        stdin().read_line(&mut line)?;
         line.pop();
         Ok(Some(Value::String(line)))
     } else {
-        Err(BadArgumentCount("input", vals.len()).into())
+        Err(BadArgumentCount("input", vals.len(), 0).into())
     }
 }
 
@@ -68,7 +88,7 @@ fn string_split(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
             Err(InvalidArgument("string-split", "target").into())
         }
     } else {
-        Err(BadArgumentCount("string-split", vals.len()).into())
+        Err(BadArgumentCount("string-split", vals.len(), 2).into())
     }
 }
 
@@ -90,7 +110,7 @@ fn array_set(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
             Err(InvalidArgument("array-set", "array").into())
         }
     } else {
-        Err(BadArgumentCount("array-set", vals.len()).into())
+        Err(BadArgumentCount("array-set", vals.len(), 3).into())
     }
 }
 
@@ -103,7 +123,7 @@ fn array_push(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
             Err(InvalidArgument("array-push", "array").into())
         }
     } else {
-        Err(BadArgumentCount("array-push", vals.len()).into())
+        Err(BadArgumentCount("array-push", vals.len(), 2).into())
     }
 }
 
@@ -119,7 +139,7 @@ fn array_pop(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
             Err(InvalidArgument("array-pop", "array").into())
         }
     } else {
-        Err(BadArgumentCount("array-pop", vals.len()).into())
+        Err(BadArgumentCount("array-pop", vals.len(), 1).into())
     }
 }
 
@@ -140,7 +160,7 @@ fn array_get(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
             Err(InvalidArgument("array-set", "array").into())
         }
     } else {
-        Err(BadArgumentCount("array-get", vals.len()).into())
+        Err(BadArgumentCount("array-get", vals.len(), 2).into())
     }
 }
 
@@ -152,7 +172,7 @@ fn array_length(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
             Err(InvalidArgument("array-length", "array").into())
         }
     } else {
-        Err(BadArgumentCount("array-length", vals.len()).into())
+        Err(BadArgumentCount("array-length", vals.len(), 1).into())
     }
 }
 
@@ -168,7 +188,7 @@ fn to_ascii(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
             Err(InvalidArgument("to-ascii", "integer").into())
         }
     } else {
-        Err(BadArgumentCount("to-ascii", vals.len()).into())
+        Err(BadArgumentCount("to-ascii", vals.len(), 1).into())
     }
 }
 
@@ -184,7 +204,7 @@ fn from_ascii(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
             Err(InvalidArgument("from-ascii", "string").into())
         }
     } else {
-        Err(BadArgumentCount("from-ascii", vals.len()).into())
+        Err(BadArgumentCount("from-ascii", vals.len(), 1).into())
     }
 }
 
@@ -197,6 +217,51 @@ fn get_args(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
                 .collect(),
         )))))
     } else {
-        Err(BadArgumentCount("get-args", vals.len()).into())
+        Err(BadArgumentCount("get-args", vals.len(), 0).into())
+    }
+}
+
+fn write_file(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
+    if vals.len() == 3 {
+        if let Value::String(path) = &vals[0] {
+            if let Value::String(contents) = &vals[1] {
+                if let Ok(mut file) = OpenOptions::new()
+                    .write(true)
+                    .create(true)
+                    .append(Value::to_bool(&vals[2]))
+                    .open(path)
+                {
+                    if let Ok(_) = write!(file, "{}", contents) {
+                        Ok(Some(Value::Integer(1)))
+                    } else {
+                        Ok(Some(Value::Integer(0)))
+                    }
+                } else {
+                    Ok(Some(Value::Integer(0)))
+                }
+            } else {
+                Err(InvalidArgument("write-file", "string").into())
+            }
+        } else {
+            Err(InvalidArgument("write-file", "string").into())
+        }
+    } else {
+        Err(BadArgumentCount("write-file", vals.len(), 3).into())
+    }
+}
+
+fn read_file(vals: Vec<Value>) -> Result<Option<Value>, Box<dyn Error>> {
+    if vals.len() == 1 {
+        if let Value::String(path) = &vals[0] {
+            if let Ok(contents) = fs::read_to_string(path) {
+                Ok(Some(Value::String(contents)))
+            } else {
+                Ok(None)
+            }
+        } else {
+            Err(InvalidArgument("read-file", "string").into())
+        }
+    } else {
+        Err(BadArgumentCount("read-file", vals.len(), 1).into())
     }
 }
diff --git a/src/value.rs b/src/value.rs
index 580e9a4..de6f563 100644
--- a/src/value.rs
+++ b/src/value.rs
@@ -19,7 +19,7 @@ impl Function {
         globals: &Context,
     ) -> Result<Option<Value>, Box<dyn Error>> {
         if args.len() != self.args.len() {
-            Err(BadArgumentCount("function", args.len()))?
+            Err(BadArgumentCount("function", args.len(), self.args.len()))?
         }
         self.args
             .iter()