diff --git a/spec/ast.txt b/spec/ast.txt
index 117f96649..5f497a3eb 100644
--- a/spec/ast.txt
+++ b/spec/ast.txt
@@ -80,7 +80,7 @@ node BoltSourceFile {
 }
 
 node BoltQualName {
-  modulePath: Vec<BoltIdentifier>,
+  modulePath: Option<Vec<BoltIdentifier>>,
   name: BoltSymbol, 
 }
 
@@ -166,7 +166,7 @@ node BoltCaseExpression > BoltExpression {
 }
 
 node BoltBlockExpression > BoltExpression {
-  statements: Vec<BoltStatement>,
+  elements: Vec<BoltFunctionBodyElement>,
 }
 
 node BoltConstantExpression > BoltExpression {
@@ -196,6 +196,8 @@ node BoltParameter {
 
 node BoltDeclaration > BoltSourceElement;
 
+node BoltTypeDeclaration > BoltSourceElement;
+
 enum BoltDeclarationModifiers {
   Mutable   = 0x1,
   Public    = 0x2,
@@ -238,7 +240,7 @@ node BoltImportDeclaration > BoltDeclaration {
   symbols: Vec<BoltImportSymbol>,
 }
 
-node BoltTraitDeclaration > BoltDeclaration {
+node BoltTraitDeclaration > BoltDeclaration, BoltTypeDeclaration {
   modifiers: BoltDeclarationModifiers,
   name: BoltIdentifier,
   typeParams: Option<Vec<BoltTypeParameter>>,
@@ -253,7 +255,7 @@ node BoltImplDeclaration > BoltDeclaration {
   elements: Vec<BoltDeclaration>,
 }
 
-node BoltTypeAliasDeclaration > BoltDeclaration {
+node BoltTypeAliasDeclaration > BoltDeclaration, BoltTypeDeclaration {
   modifiers: BoltDeclarationModifiers,
   name: BoltIdentifier,
   typeParams: Option<Vec<BoltTypeParameter>>,
@@ -267,9 +269,9 @@ node BoltRecordField > BoltRecordMember {
   type: BoltTypeExpression,
 }
 
-node BoltRecordDeclaration > BoltDeclaration {
+node BoltRecordDeclaration > BoltDeclaration, BoltTypeDeclaration {
   modifiers: BoltDeclarationModifiers,
-  name: BoltQualName,
+  name: BoltIdentifier,
   typeParms: Option<Vec<BoltTypeParameter>>,
   members: Option<Vec<BoltRecordMember>>,
 }
diff --git a/src/checker.ts b/src/checker.ts
index 5028c534d..9fc55d4ca 100644
--- a/src/checker.ts
+++ b/src/checker.ts
@@ -23,9 +23,18 @@
  * Note that the `pub`-keyword is not present on `MyType1`.
  */
 
-import {Syntax, SyntaxKind, BoltReferenceExpression, BoltDeclaration, BoltSourceFile, BoltSyntax, BoltReferenceTypeExpression, BoltTypeDeclaration} from "./ast";
-import {FastStringMap} from "./util";
-import {DiagnosticPrinter, E_TYPES_NOT_ASSIGNABLE, E_TYPE_DECLARATION_NOT_FOUND, E_DECLARATION_NOT_FOUND} from "./diagnostics";
+import {Syntax, SyntaxKind, BoltReferenceExpression, BoltDeclaration, BoltSourceFile, BoltSyntax, BoltReferenceTypeExpression, BoltTypeDeclaration, BoltExpression, BoltFunctionDeclaration, BoltFunctionBodyElement, kindToString, createBoltReferenceTypeExpression, createBoltIdentifier} from "./ast";
+import {FastStringMap, memoize, assert} from "./util";
+import {
+  DiagnosticPrinter,
+  E_TYPES_NOT_ASSIGNABLE,
+  E_TOO_MANY_ARGUMENTS_FOR_FUNCTION_CALL,
+  E_TOO_FEW_ARGUMENTS_FOR_FUNCTION_CALL,
+  E_TYPE_DECLARATION_NOT_FOUND,
+  E_DECLARATION_NOT_FOUND,
+  E_INVALID_ARGUMENTS
+} from "./diagnostics";
+import {createAnyType, isOpaqueType, createOpaqueType, Type} from "./types";
 
 interface SymbolInfo {
   declarations: BoltDeclaration[];
@@ -93,6 +102,153 @@ export class TypeChecker {
       }
     }
 
+    const callExps = node.findAllChildrenOfKind(SyntaxKind.BoltCallExpression);
+
+    for (const callExp of callExps) {
+
+      const fnDecls = this.getAllFunctionsInExpression(callExp.operator);
+
+      for (const fnDecl of fnDecls) {
+
+        if (fnDecl.params.length > callExp.operands.length) {
+          this.diagnostics.add({
+            message: E_TOO_FEW_ARGUMENTS_FOR_FUNCTION_CALL,
+            args: { expected: fnDecl.params.length, actual: callExp.operands.length },
+            severity: 'error',
+            node: callExp,
+          })
+        }
+
+        if (fnDecl.params.length < callExp.operands.length) {
+          this.diagnostics.add({
+            message: E_TOO_MANY_ARGUMENTS_FOR_FUNCTION_CALL,
+            args: { expected: fnDecl.params.length, actual: callExp.operands.length },
+            severity: 'error',
+            node: callExp,
+          })
+        }
+
+        const paramCount = fnDecl.params.length;
+        for (let i = 0; i < paramCount; i++) {
+          const arg = callExp.operands[i];
+          const param = fnDecl.params[i];
+          let argType = this.getTypeOfNode(arg);
+          let paramType = this.getTypeOfNode(param);
+          if (!this.isTypeAssignableTo(argType, paramType)) {
+            this.diagnostics.add({
+              message: E_INVALID_ARGUMENTS,
+              severity: 'error',
+              args: { name: fnDecl.name.text },
+              node: arg,
+            });
+          }
+
+        }
+
+      }
+
+    }
+
+  }
+
+  private resolveType(name: string, node: BoltSyntax): Type | null {
+    const sym = this.findSymbolInTypeScopeOf(name, this.getTypeScopeSurroundingNode(node))
+    if (sym === null) {
+      return null;
+    }
+    return this.getTypeOfNode(sym.declarations[0]);
+  }
+
+  @memoize
+  private getTypeOfNode(node: BoltSyntax): Type {
+    switch (node.kind) {
+      case SyntaxKind.BoltReferenceTypeExpression:
+      {
+        const referenced = this.resolveTypeReferenceExpression(node);
+        if (referenced === null) {
+          return createAnyType();
+        }
+        return this.getTypeOfNode(referenced);
+      }
+      case SyntaxKind.BoltRecordDeclaration:
+      {
+        if (node.members === null) {
+          return createOpaqueType();
+        }
+        // TODO
+        throw new Error(`Not yet implemented.`);
+      }
+      case SyntaxKind.BoltParameter:
+      {
+        let type: Type = createAnyType();
+        if (node.type !== null) {
+          type = this.getTypeOfNode(node.type);
+        }
+        return type;
+      }
+      case SyntaxKind.BoltConstantExpression:
+      {
+        let type;
+        if (typeof node.value === 'string') {
+          type = this.resolveType('String', node)!;
+        } else if (typeof node.value === 'boolean') {
+          type = this.resolveType('bool', node)!;
+        } else if (typeof node.value === 'number') {
+          type = this.resolveType('int32', node)!;
+        } else {
+          throw new Error(`Could not derive type of constant expression.`);
+        }
+        assert(type !== null);
+        return type;
+      }
+      default:
+          throw new Error(`Could not derive type of node ${kindToString(node.kind)}.`);
+    }
+  }
+
+  private isTypeAssignableTo(left: Type, right: Type): boolean {
+    if (isOpaqueType(left) &&  isOpaqueType(right)) {
+      return left === right;
+    }
+    return false;
+  }
+
+  private getAllFunctionsInExpression(node: BoltExpression): BoltFunctionDeclaration[] {
+
+    const self = this;
+
+    const results: BoltFunctionDeclaration[] = [];
+    visitExpression(node);
+    return results;
+
+    function visitExpression(node: BoltExpression) {
+      switch (node.kind) {
+        case SyntaxKind.BoltReferenceExpression:
+          const resolved = self.resolveReferenceExpression(node);
+          if (resolved !== null) {
+            visitFunctionBodyElement(resolved);
+          }
+          break;
+        default:
+          throw new Error(`Unexpected node type ${kindToString(node.kind)}`);
+      }
+    }
+
+    function visitFunctionBodyElement(node: BoltFunctionBodyElement) {
+      switch (node.kind) {
+        case SyntaxKind.BoltFunctionDeclaration:
+          results.push(node);
+          break;
+        case SyntaxKind.BoltVariableDeclaration:
+          if (node.value !== null) {
+            visitExpression(node.value);
+          }
+          break;
+        default:
+          throw new Error(`Unexpected node type ${kindToString(node.kind)}`);
+      }
+    }
+
   }
 
   public registerSourceFile(node: BoltSourceFile): void {
@@ -105,10 +261,12 @@ export class TypeChecker {
 
       case SyntaxKind.BoltSourceFile:
       case SyntaxKind.BoltModule:
+      {
         for (const element of node.elements) {
           this.addAllSymbolsInNode(element);
         }
         break;
+      }
 
       case SyntaxKind.BoltFunctionDeclaration:
       {
diff --git a/src/diagnostics.ts b/src/diagnostics.ts
index f982e2b17..6c0446d8c 100644
--- a/src/diagnostics.ts
+++ b/src/diagnostics.ts
@@ -6,6 +6,9 @@ import {format, MapLike, FormatArg} from "./util";
 export const E_TYPE_DECLARATION_NOT_FOUND = "A type declaration named '{name}' was not found."
 export const E_DECLARATION_NOT_FOUND = "Reference to an undefined declaration '{name}'.";
 export const E_TYPES_NOT_ASSIGNABLE = "Types {left} and {right} are not assignable.";
+export const E_TOO_FEW_ARGUMENTS_FOR_FUNCTION_CALL = "Too few arguments for function call. Expected {expected} but got {actual}.";
+export const E_TOO_MANY_ARGUMENTS_FOR_FUNCTION_CALL = "Too many arguments for function call. Expected {expected} but got {actual}.";
+export const E_INVALID_ARGUMENTS = "Invalid arguments passed to function '{name}'."
 
 export interface Diagnostic {
   message: string;
diff --git a/src/types.ts b/src/types.ts
index 1c4ef0019..2a9a584a8 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -32,6 +32,10 @@ export function isOpaqueType(value: any): value is OpaqueType {
   return value.kind === TypeKind.OpaqueType;
 }
 
+export function createOpaqueType(): OpaqueType {
+  return new OpaqueType();
+}
+
 export class AnyType extends TypeBase {
   kind: TypeKind.AnyType = TypeKind.AnyType;
 }
diff --git a/src/util.ts b/src/util.ts
index 0dfd2059d..939c278c9 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -468,8 +468,9 @@ export function format(message: string, data: MapLike<FormatArg>) {
       if (ch === '}') {
         out += data[name]!.toString();
         reset();
+      } else {
+        name += ch;
       }
-      name += ch
     } else {
       if (ch === '{') {
         insideParam = true;