WIP on strictNullChecks - this one is tough

Author: joews

src/interpreter.ts

@@ -21,7 +21,7 @@ export type Visitor = (node: TypedNode, env: RuntimeEnv) => InterpreterResult
 // Visit each of `nodes` in order, returning the result
 // and environment of the last node.
 function visitSerial (nodes: TypedNode[], rootEnv: RuntimeEnv): InterpreterResult {
-  const initialState: [TypedNode, RuntimeEnv] = [null, rootEnv]
+  const initialState: [TypedNode, RuntimeEnv] = [nodes[0], rootEnv]
   return nodes.reduce(([, env], node) => (
     visit(node, env)
   ), initialState)

src/type-checker.ts

@@ -8,7 +8,8 @@ import {
   AstDestructuredArrayNode, AstFunctionNode, AstIfNode,
   TypedAst, TypedNode, TypedProgramNode, TypedDefNode, TypedNameNode,
   TypedNumeralNode, TypedBooleanNode, TypedStringNode, TypedCallNode, TypedArrayNode,
-  TypedDestructuredArrayNode, TypedFunctionNode, TypedFunctionClauseNode, TypedIfNode
+  TypedDestructuredArrayNode, TypedFunctionNode, TypedFunctionClauseNode, TypedIfNode,
+  NoBoundValueYetNode
 } from './node-types'
 
 import {
@@ -294,6 +295,8 @@ function fresh (type: Type, nonGeneric: Set<Type>) {
 
       // FIXME find out how to do this with type safety, given type erasure.
       return (pruned.constructor as any).of(pruned.name, freshTypeArgs)
+    } else {
+      return t
     }
   }
 

src/types.ts

@@ -94,7 +94,7 @@ export class TypeVariable extends Type {
   instance: Type
 
   constructor () {
-    super(null)
+    super('')
     this.id = TypeVariable.nextId ++
   }
 

src/unify.ts

@@ -3,7 +3,7 @@ import { isArray, isEqual } from './array'
 import { zip } from './util'
 import { AstNode, AstLiteralNode, AstDestructuredArrayNode, Value, isAstLiteralNode } from './node-types'
 
-// FIXME tagged union - with didMatch: false, there are never bindings.
+// FIXME tagged union - with didMatch: false, there are never bindings (hence binding | null return types)
 type binding = { [name: string]: Value }
 
 interface unification {
@@ -29,7 +29,7 @@ export default function unify (patterns: AstNode[], values: Value[]): unificatio
   return didMatch(bindings)
 }
 
-function unifyOne (pattern: AstNode, value: Value): binding {
+function unifyOne (pattern: AstNode, value: Value): binding | null {
   if (isAstLiteralNode(pattern) && pattern.value === value) {
     // the pattern matched, but there is nothing to bind
     return {}
@@ -54,7 +54,7 @@ function unifyOne (pattern: AstNode, value: Value): binding {
 }
 
 // TODO this will need to change when Array is a wrapped type
-function destructure ({ head, tail }: AstDestructuredArrayNode, values: Value[]): binding {
+function destructure ({ head, tail }: AstDestructuredArrayNode, values: Value[]): binding | null {
   if (values.length === 0) {
     throw new PeachError(`Empty arrays cannot be destructured because they don't have a head`)
   }

tsconfig.json

@@ -7,6 +7,7 @@
         "target": "es2017",
         "sourceMap": true,
         "noImplicitAny": true,
+        "strictNullChecks": true,
         "noImplicitThis": true,
         "alwaysStrict": true
     },