diff --git a/src/ast/mod.rs b/src/ast/mod.rs
index 36b87aa..e568a8f 100644
--- a/src/ast/mod.rs
+++ b/src/ast/mod.rs
@@ -32,6 +32,8 @@ use crate::build::scope::Scope;
 use crate::build::Val;
 use crate::error::{BuildError, ErrorType::TypeFail};
 
+use self::walk::Walker;
+
 pub mod printer;
 pub mod rewrite;
 pub mod typecheck;
@@ -272,8 +274,8 @@ value_enum!(
 );
 
 impl Shape {
-    pub fn merge(&self, compare: &Shape) -> Option<Self> {
-        match (self, compare) {
+    pub fn merge(&self, right: &Shape) -> Option<Self> {
+        match (self, right) {
             (Shape::Str(_), Shape::Str(_))
             | (Shape::Symbol(_), Shape::Symbol(_))
             | (Shape::Boolean(_), Shape::Boolean(_))
@@ -775,7 +777,7 @@ fn normalize_path(p: PathBuf) -> PathBuf {
     return normalized;
 }
 
-impl walk::Walker for Rewriter {
+impl Walker for Rewriter {
     fn walk_statement_list(&mut self, stmts: Vec<&mut Statement>) {
         for v in stmts {
             self.walk_statement(v);
@@ -1046,12 +1048,26 @@ impl Expression {
                 CastType::Float => Shape::Float(PositionedItem::new(0.0, def.pos.clone())),
                 CastType::Bool => Shape::Boolean(PositionedItem::new(true, def.pos.clone())),
             },
-            Expression::Import(def) => {
-                // FIXME(jwall): What should this do?
-                Shape::Symbol(PositionedItem::new(
-                    def.path.fragment.clone(),
-                    def.path.pos.clone(),
-                ))
+            Expression::Import(def) => Shape::Symbol(PositionedItem::new(
+                def.path.fragment.clone(),
+                def.path.pos.clone(),
+            )),
+            Expression::Binary(def) => {
+                let left_shape = def.left.derive_shape()?;
+                let right_shape = def.right.derive_shape()?;
+                match left_shape.merge(&right_shape) {
+                    Some(shape) => shape,
+                    None => {
+                        return Err(BuildError::new(
+                            format!(
+                                "Expected {} value on right hand side of expression but got: {}",
+                                left_shape.type_name(),
+                                right_shape.type_name()
+                            ),
+                            TypeFail,
+                        ));
+                    }
+                }
             }
             _ => Shape::Empty(Position::new(0, 0, 0)),
         };
diff --git a/src/ast/rewrite.rs b/src/ast/rewrite.rs
index 9fd33c8..9153ba7 100644
--- a/src/ast/rewrite.rs
+++ b/src/ast/rewrite.rs
@@ -13,7 +13,7 @@
 // limitations under the License.
 use std::path::PathBuf;
 
-use crate::ast::walk::{Visitor, Walker};
+use crate::ast::walk::Visitor;
 use crate::ast::Expression;
 
 pub struct Rewriter {
diff --git a/src/ast/test.rs b/src/ast/test.rs
index efb626e..e508881 100644
--- a/src/ast/test.rs
+++ b/src/ast/test.rs
@@ -154,6 +154,10 @@ fn derive_shape_expressions() {
             "bool(\"true\");",
             Shape::Boolean(PositionedItem::new(true, Position::new(0, 0, 0))),
         ),
+        (
+            "1 + 1;",
+            Shape::Int(PositionedItem::new(1, Position::new(1, 1, 0))),
+        ),
     ];
 
     for (expr, shape) in expr_cases {
diff --git a/src/ast/typecheck/mod.rs b/src/ast/typecheck/mod.rs
index a2b8717..aabff19 100644
--- a/src/ast/typecheck/mod.rs
+++ b/src/ast/typecheck/mod.rs
@@ -13,7 +13,7 @@
 // limitations under the License.
 
 //! Implements typechecking for the parsed ucg AST.
-
+// FIXME(jwall): This probably just needs to disappear now.
 use std::collections::BTreeMap;
 
 use crate::ast::walk::Visitor;
diff --git a/src/build/opcode/environment.rs b/src/build/opcode/environment.rs
index d4030db..5e8dc66 100644
--- a/src/build/opcode/environment.rs
+++ b/src/build/opcode/environment.rs
@@ -35,6 +35,7 @@ where
     Stderr: Write + Clone,
 {
     pub val_cache: BTreeMap<String, Rc<Value>>,
+    // TODO implement a shape cache here.
     pub op_cache: cache::Ops,
     pub converter_registry: ConverterRegistry,
     pub importer_registry: ImporterRegistry,
diff --git a/src/build/opcode/translate.rs b/src/build/opcode/translate.rs
index aed360c..016671a 100644
--- a/src/build/opcode/translate.rs
+++ b/src/build/opcode/translate.rs
@@ -18,7 +18,7 @@ use crate::ast::rewrite::Rewriter;
 use crate::ast::walk::Walker;
 use crate::ast::{
     BinaryExprType, BinaryOpDef, Expression, FormatArgs, FuncOpDef, Position, PositionedItem,
-    SelectDef, Statement, TemplatePart, Token, TokenType, Value,
+    SelectDef, Shape, Statement, TemplatePart, Token, TokenType, Value,
 };
 use crate::build::format::{ExpressionTemplate, SimpleTemplate, TemplateParser};
 use crate::build::opcode::Primitive;
@@ -30,6 +30,7 @@ pub struct AST();
 pub struct OpsMap {
     pub ops: Vec<Op>,
     pub pos: Vec<Position>,
+    pub shape_map: BTreeMap<String, Shape>,
     pub links: BTreeMap<String, Position>,
 }
 
@@ -38,6 +39,8 @@ impl OpsMap {
         OpsMap {
             ops: Vec::new(),
             pos: Vec::new(),
+            // TODO(jwall): Populate shapes when translating into opcodes.
+            shape_map: BTreeMap::new(),
             links: BTreeMap::new(),
         }
     }