diff --git a/.gitignore b/.gitignore
index 03a5621e4..e7669ef01 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,4 +4,6 @@ example.tf
 terraform.tfplan
 terraform.tfstate
 bin/
+config/y.go
+config/y.output
 vendor/
diff --git a/config/expr.y b/config/expr.y
new file mode 100644
index 000000000..58e5fac67
--- /dev/null
+++ b/config/expr.y
@@ -0,0 +1,84 @@
+// This is the yacc input for creating the parser for interpolation
+// expressions in Go.
+
+// To build it:
+//
+// go tool yacc -p "expr" expr.y (produces y.go)
+//
+
+%{
+package config
+
+import (
+	"fmt"
+)
+
+%}
+
+%union {
+	expr Interpolation
+    str string
+	variable InterpolatedVariable
+	args []Interpolation
+}
+
+%type	<args> args
+%type   <expr> expr
+%type   <str> string
+%type   <variable> variable
+
+%token  <str> STRING IDENTIFIER
+%token	<str> COMMA LEFTPAREN RIGHTPAREN
+
+%%
+
+top:
+	expr
+	{
+		fmt.Printf("%#v", $1)
+	}
+
+expr:
+	string
+	{
+		$$ = &LiteralInterpolation{Literal: $1}
+	}
+|	variable
+	{
+		$$ = &VariableInterpolation{Variable: $1}
+	}
+|	IDENTIFIER LEFTPAREN args RIGHTPAREN
+	{
+		$$ = &FunctionInterpolation{Func: $1, Args: $3}
+	}
+
+args:
+	{
+		$$ = nil
+	}
+|	expr COMMA expr
+	{
+		$$ = append($$, $1, $3)
+	}
+|	expr
+	{
+		$$ = append($$, $1)
+	}
+
+string:
+	STRING
+	{
+		$$ = $1
+	}
+
+variable:
+	IDENTIFIER
+	{
+		var err error
+		$$, err = NewInterpolatedVariable($1)
+		if err != nil {
+			panic(err)
+		}
+	}
+
+%%
diff --git a/config/expr_lex.go b/config/expr_lex.go
new file mode 100644
index 000000000..5e8404431
--- /dev/null
+++ b/config/expr_lex.go
@@ -0,0 +1,130 @@
+package config
+
+import (
+	"bytes"
+	"log"
+	"unicode"
+	"unicode/utf8"
+)
+
+// The parser expects the lexer to return 0 on EOF.
+const lexEOF = 0
+
+// The parser uses the type <prefix>Lex as a lexer.  It must provide
+// the methods Lex(*<prefix>SymType) int and Error(string).
+type exprLex struct {
+	input string
+	pos   int
+	width int
+}
+
+// The parser calls this method to get each new token.
+func (x *exprLex) Lex(yylval *exprSymType) int {
+	for {
+		c := x.next()
+		if c == lexEOF {
+			return lexEOF
+		}
+
+		// Ignore all whitespace
+		if unicode.IsSpace(c) {
+			continue
+		}
+
+		switch c {
+		case '"':
+			return x.lexString(yylval)
+		case ',':
+			return COMMA
+		case '(':
+			return LEFTPAREN
+		case ')':
+			return RIGHTPAREN
+		default:
+			x.backup()
+			return x.lexId(yylval)
+		}
+	}
+}
+
+func (x *exprLex) lexId(yylval *exprSymType) int {
+	var b bytes.Buffer
+	for {
+		c := x.next()
+		if c == lexEOF {
+			break
+		}
+
+		// If this isn't a character we want in an ID, return out.
+		// One day we should make this a regexp.
+		if c != '_' &&
+			c != '.' &&
+			c != '*' &&
+			!unicode.IsLetter(c) &&
+			!unicode.IsNumber(c) {
+			x.backup()
+			break
+		}
+
+		if _, err := b.WriteRune(c); err != nil {
+			log.Printf("ERR: %s", err)
+			return lexEOF
+		}
+	}
+
+	yylval.str = b.String()
+	return IDENTIFIER
+}
+
+func (x *exprLex) lexString(yylval *exprSymType) int {
+	var b bytes.Buffer
+	for {
+		c := x.next()
+		if c == lexEOF {
+			break
+		}
+
+		// String end
+		if c == '"' {
+			break
+		}
+
+		if _, err := b.WriteRune(c); err != nil {
+			log.Printf("ERR: %s", err)
+			return lexEOF
+		}
+	}
+
+	yylval.str = b.String()
+	return STRING
+}
+
+// Return the next rune for the lexer.
+func (x *exprLex) next() rune {
+	if int(x.pos) >= len(x.input) {
+		x.width = 0
+		return lexEOF
+	}
+
+	r, w := utf8.DecodeRuneInString(x.input[x.pos:])
+	x.width = w
+	x.pos += x.width
+	return r
+}
+
+// peek returns but does not consume the next rune in the input
+func (x *exprLex) peek() rune {
+	r := x.next()
+	x.backup()
+	return r
+}
+
+// backup steps back one rune. Can only be called once per next.
+func (x *exprLex) backup() {
+	x.pos -= x.width
+}
+
+// The parser calls this method on a parse error.
+func (x *exprLex) Error(s string) {
+	log.Printf("parse error: %s", s)
+}
diff --git a/config/expr_test.go b/config/expr_test.go
new file mode 100644
index 000000000..68a6b5fd4
--- /dev/null
+++ b/config/expr_test.go
@@ -0,0 +1,9 @@
+package config
+
+import (
+	"testing"
+)
+
+func TestExprParse(t *testing.T) {
+	exprParse(&exprLex{input: `lookup(var.foo)`})
+}
diff --git a/config/interpolate.go b/config/interpolate.go
index 935ff2e8f..64cb90df0 100644
--- a/config/interpolate.go
+++ b/config/interpolate.go
@@ -37,8 +37,8 @@ type InterpolatedVariable interface {
 // FunctionInterpolation is an Interpolation that executes a function
 // with some variable number of arguments to generate a value.
 type FunctionInterpolation struct {
-	Func InterpolationFunc
-	Args []InterpolatedVariable
+	Func string
+	Args []Interpolation
 
 	key string
 }