diff --git a/PRIMITIVES.md b/PRIMITIVES.md
index 29abaf2..6583793 100644
--- a/PRIMITIVES.md
+++ b/PRIMITIVES.md
@@ -84,7 +84,7 @@ Things you'll find here include:
 * `chr`
   * Return the ASCII character of the given number.
 * `cons`
-  * Join two specified lists.
+  * Add the element to the start of the given (potentialy empty) list.
 * `contains?`
   * Does the specified hash contain the given key?
 * `date`
@@ -108,6 +108,8 @@ Things you'll find here include:
   * Return the contents of the given file, as a string.
 * `file:stat`
   * Return details of the given path.
+* `file:write`
+  * Write the specified content to the provided path.
 * `gensym`
   * Generate, and return, a unique symbol.  Useful for macro definitions.
 * `get`
@@ -188,8 +190,11 @@ Functions here include:
 * `and`
   * Logical operator, are all elements true?
 * `append`
+  * Append the given entry to the specified list.
 * `apply`
+  * Return the result of calling the specified function on every element in the supplied list, as a list.
 * `apply-hash`
+  * Apply the given function to everyu key in the specified hash, and return the result as list.
 * `boolean?`
   * Is the given thing a boolean?
 * `butlast`
@@ -223,6 +228,11 @@ Functions here include:
   * Return the size of the path, from the information provided by `(file:stat)`.
 * `file:stat:uid`
   * Return the UID of the path, from the information provided by `(file:stat)`.
+* `file:which`
+  * Locate the specified binary's location, upon the users' PATH.
+  * NOTE: This is almost certainly Unix/Linux/Darwin only, and will fail upon Windows systems.
+* `file:write`
+  * Write the specified content to the given path.
 * `filter`
   * Remove every element from the given list, unless the function returns true.
 * `first`
@@ -273,6 +283,7 @@ Functions here include:
 * `range`
   * Return a list of numbers between the given start/end, using the specified step-size.
 * `reduce`
+  * Our reduce function, with the list, function and accumulator.
 * `repeat`
   * Run the given body N times.
 * `repeated`
diff --git a/builtins/builtins.go b/builtins/builtins.go
index 1e6cf58..c8e34c1 100644
--- a/builtins/builtins.go
+++ b/builtins/builtins.go
@@ -114,6 +114,7 @@ func PopulateEnvironment(env *env.Environment) {
 	env.Set("file:lines", &primitive.Procedure{F: fileLinesFn, Help: helpMap["file:lines"], Args: []primitive.Symbol{primitive.Symbol("path")}})
 	env.Set("file:read", &primitive.Procedure{F: fileReadFn, Help: helpMap["file:read"], Args: []primitive.Symbol{primitive.Symbol("path")}})
 	env.Set("file:stat", &primitive.Procedure{F: fileStatFn, Help: helpMap["file:stat"], Args: []primitive.Symbol{primitive.Symbol("path")}})
+	env.Set("file:write", &primitive.Procedure{F: fileWriteFn, Help: helpMap["file:write"], Args: []primitive.Symbol{primitive.Symbol("path"), primitive.Symbol("content")}})
 	env.Set("file?", &primitive.Procedure{F: fileFn, Help: helpMap["file?"], Args: []primitive.Symbol{primitive.Symbol("path")}})
 	env.Set("gensym", &primitive.Procedure{F: gensymFn, Help: helpMap["gensym"]})
 	env.Set("get", &primitive.Procedure{F: getFn, Help: helpMap["get"], Args: []primitive.Symbol{primitive.Symbol("hash"), primitive.Symbol("key")}})
@@ -616,6 +617,31 @@ func fileStatFn(env *env.Environment, args []primitive.Primitive) primitive.Prim
 	return res
 }
 
+// fileWriteFn implements file:write
+func fileWriteFn(env *env.Environment, args []primitive.Primitive) primitive.Primitive {
+	// We need two arguments
+	if len(args) != 2 {
+		return primitive.ArityError()
+	}
+
+	// Path is a string
+	path, ok := args[0].(primitive.String)
+	if !ok {
+		return primitive.Error("argument not a string")
+	}
+
+	// Content is a string
+	content, ok := args[1].(primitive.String)
+	if !ok {
+		return primitive.Error("argument not a string")
+	}
+
+	err := os.WriteFile(path.ToString(), []byte(content.ToString()), 0777)
+	if err != nil {
+		return primitive.Error(fmt.Sprintf("failed to write to %s:%s", path.ToString(), err))
+	}
+	return primitive.Nil{}
+}
 // gensymFn is the implementation of (gensym ..)
 func gensymFn(env *env.Environment, args []primitive.Primitive) primitive.Primitive {
 	// symbol characters
diff --git a/builtins/builtins_test.go b/builtins/builtins_test.go
index ac7698d..1081e76 100644
--- a/builtins/builtins_test.go
+++ b/builtins/builtins_test.go
@@ -1223,6 +1223,84 @@ func TestFileStat(t *testing.T) {
 	}
 }
 
+// TestFileWrite tests file:write
+func TestFileWrite(t *testing.T) {
+
+	// calling with no argument
+	out := fileWriteFn(ENV, []primitive.Primitive{})
+
+	// Will lead to an error
+	_, ok := out.(primitive.Error)
+	if !ok {
+		t.Fatalf("expected error, got %v", out)
+
+	}
+
+	// Call with the wrong type
+	out = fileWriteFn(ENV, []primitive.Primitive{
+		primitive.Number(3),
+		primitive.String("cake")})
+
+	_, ok = out.(primitive.Error)
+	if !ok {
+		t.Fatalf("expected error, got %v", out)
+	}
+
+	// Call with the wrong type
+	out = fileWriteFn(ENV, []primitive.Primitive{
+		primitive.String("/tmp/blah.txt"),
+		primitive.Number(3)})
+
+	_, ok = out.(primitive.Error)
+	if !ok {
+		t.Fatalf("expected error, got %v", out)
+	}
+
+	// Now we can try to write to something real
+	// Create a temporary file to write to
+	tmpfile, err := os.CreateTemp("", "exists")
+	if err != nil {
+		t.Fatalf("failed to create a temporary file")
+	}
+	defer os.Remove(tmpfile.Name())
+	// Try to write there
+	result := fileWriteFn(ENV, []primitive.Primitive{
+		primitive.String(tmpfile.Name()),
+		primitive.String("cake")})
+
+	// Should be no errors
+	_, ok = result.(primitive.Nil)
+	if !ok {
+		t.Fatalf("expected nil, got %v", out)
+	}
+
+	// Now create a temmporary directory, and use that
+	// as the destination - which will fail once it is
+	// removed
+	path, err2 := os.MkdirTemp("", "directory_")
+	if err2 != nil {
+		t.Fatalf("failed to create temporary directory")
+	}
+
+	// path beneath the temporary directory
+	target := filepath.Join(path, "foo.txt")
+
+	// remove the direcotry
+	os.RemoveAll(path)
+
+
+	failure := fileWriteFn(ENV, []primitive.Primitive{
+		primitive.String(target),
+		primitive.String("cake")})
+
+	// Should be no errors
+	_, ok = failure.(primitive.Error)
+	if !ok {
+		t.Fatalf("expected failure writing beneath a directory which was removed, got %v", out)
+	}
+
+}
+
 // TestGenSym tests gensym
 func TestGenSym(t *testing.T) {
 
diff --git a/builtins/help.txt b/builtins/help.txt
index dd84047..7087193 100644
--- a/builtins/help.txt
+++ b/builtins/help.txt
@@ -109,16 +109,14 @@ file:lines
 
 file:lines returns the contents of the given file, as a list of lines.
 
-See also: file:read
+See also: file:read, file:write
 %%
 file:read
 
 file:read returns the contents of the given file, as a string.
 
-See also: file:lines
+See also: file:lines, file:write
 %%
-
-
 file:stat
 
 file:stat returns a list containing details of the given file/directory,
@@ -129,6 +127,12 @@ The return value is (NAME SIZE UID GID MODE).
 See also: file:stat:gid file:stat:mode file:stat:size file:stat:uid
 Example: (print (file:stat "/etc/passwd"))
 %%
+file:write
+
+Write the given content to the specified path.
+
+Example: (file:write "/tmp/test.txt" "I like cake.")
+%%
 gensym
 
 gensym returns a symbol which is guaranteed to be unique.  It is primarily