[#8] Add proper exception handling

.gitignore

@@ -4,3 +4,4 @@ dist/
 lib/
 storybook-static/
 docs/
+coverage/

src/exceptions.ts

@@ -0,0 +1,6 @@
+export class InferenceError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = 'InferenceError';
+  }
+}

src/helper.ts

@@ -1,6 +1,7 @@
 // Copyright (c) 2023 Adam Jones
 //
 // SPDX-License-Identifier: MIT
+import {InferenceError} from './exceptions';
 import {
   Context,
   ExplainPath,
@@ -56,7 +57,7 @@ function apply(
     return {...value, sigma: apply(s, value.sigma)};
   }
   ((_: never): never => {
-    throw new Error('Unknown argument passed to substitution');
+    throw new InferenceError('Unknown argument passed to substitution');
   })(value);
 }
 
@@ -96,7 +97,7 @@ export const instantiate = (
   }
 
   ((_: never): never => {
-    throw new Error('Unknown type passed to instantiate');
+    throw new InferenceError('Unknown type passed to instantiate');
   })(type);
 };
 
@@ -133,7 +134,7 @@ const freeVars = (value: PolyType | Context): string[] => {
   }
 
   ((_: never): never => {
-    throw new Error('Unknown argument passed to substitution');
+    throw new InferenceError('Unknown argument passed to substitution');
   })(value);
 };
 
@@ -152,7 +153,7 @@ export const unify = (
 
   if (type1.type === 'ty-var') {
     if (contains(type2, type1))
-      throw new Error(`Infinite type detected: ${type1} occurs in ${type2}`);
+      throw new InferenceError(`Infinite type detected: ${type1} occurs in ${type2}`);
 
     if (type2.type === 'ty-var') {
       // var with other name -> explain
@@ -164,7 +165,7 @@ export const unify = (
       type1.explain = [type2, {type: 'ExplainInstan', path: path1, expr}];
     } else {
       ((_: never): never => {
-        throw new Error('Unknown argument passed to unify');
+        throw new InferenceError('Unknown argument passed to unify');
       })(type2);
     }
     return makeSubstitution({
@@ -178,11 +179,13 @@ export const unify = (
 
   if (type1.C !== type2.C) {
     const msg = formatUnificationError(type1, type2, expr, path1, path2);
-    throw new Error(msg);
+    throw new InferenceError(msg);
   }
 
   if (type1.mus.length !== type2.mus.length) {
-    throw new Error(`Could not unify types (different argument lengths): ${type1} and ${type2}`);
+    throw new InferenceError(
+      `Could not unify types (different argument lengths): ${type1} and ${type2}`
+    );
   }
 
   let s: Substitution = makeSubstitution({});
@@ -208,7 +211,7 @@ const formatUnificationError = (
     }, but it is a ${type2.C}`;
     return msg;
   }
-  throw new Error(`Unexpected expression type ${expr}`);
+  throw new InferenceError(`Unexpected expression type ${expr}`);
 };
 
 export const reprExpression = (expr: Expression): string => {
@@ -220,7 +223,7 @@ export const reprExpression = (expr: Expression): string => {
   if (expr.type === 'let')
     return `"${expr.x}" = ${reprExpression(expr.e1)} in ${reprExpression(expr.e2)}`;
   ((_: never): never => {
-    throw new Error(`Unexpected expression type ${expr}`);
+    throw new InferenceError(`Unexpected expression type ${expr}`);
   })(expr);
 };
 
@@ -246,6 +249,6 @@ const contains = (value: MonoType, type2: TypeVariable): boolean => {
   }
 
   ((_: never): never => {
-    throw new Error('Unknown argument passed to substitution');
+    throw new InferenceError('Unknown argument passed to substitution');
   })(value);
 };

src/index.ts

@@ -1,10 +1,16 @@
+import type {Substitution} from './helper';
 import type {Context} from './models';
 import type {JSONValue} from './parser';
 import {defaultContext, parseContext, parseJsonLogicExpression} from './parser';
 import {W} from './w';
 
 export {defaultContext};
 
+interface InferenceResult {
+  resultType: string;
+  intermediateVariables: Substitution['raw'];
+}
+
 /**
  * @beta
  * @param jsonLogic - JsonLogic expression/rule
@@ -16,14 +22,12 @@ export const infer = (
   jsonLogic: JSONValue,
   data: JSONValue,
   context: Context = defaultContext
-): string => {
+): InferenceResult => {
   const typeenv = parseContext(data, context);
-  try {
-    const [subsitution, t] = W(...parseJsonLogicExpression(jsonLogic, typeenv));
-    const type: string = 'C' in t ? t.C : JSON.stringify([t.a, Object.keys(subsitution.raw)]);
-    const ctx: string = JSON.stringify(subsitution.raw);
-    return `result type: ${type}\n(intermediate) variables:${ctx}`;
-  } catch (error) {
-    return error.toString();
-  }
+  const [subsitution, t] = W(...parseJsonLogicExpression(jsonLogic, typeenv));
+  const type: string = 'C' in t ? t.C : JSON.stringify([t.a, Object.keys(subsitution.raw)]);
+  return {
+    resultType: type,
+    intermediateVariables: subsitution.raw,
+  };
 };

src/m.ts

@@ -1,6 +1,7 @@
 // Copyright (c) 2023 Adam Jones
 //
 // SPDX-License-Identifier: MIT
+import {InferenceError} from './exceptions';
 import {Substitution, generalise, instantiate, newTypeVar, unify} from './helper';
 import {Context, Expression, MonoType, makeContext} from './models';
 
@@ -9,7 +10,7 @@ export const M = (typEnv: Context, expr: Expression, type: MonoType): Substituti
     console.log(`Variable ${expr.x}: expected to have type ${JSON.stringify(type)}`);
 
     const value = typEnv[expr.x];
-    if (value === undefined) throw new Error(`Undefined variable: ${expr.x}`);
+    if (value === undefined) throw new InferenceError(`Undefined variable: ${expr.x}`);
     return unify(type, instantiate(value), expr);
   }
 
@@ -69,5 +70,5 @@ export const M = (typEnv: Context, expr: Expression, type: MonoType): Substituti
     return s2(s1);
   }
 
-  throw new Error('Unknown expression type');
+  throw new InferenceError('Unknown expression type');
 };

src/parser.ts

@@ -1,3 +1,4 @@
+import {InferenceError} from './exceptions';
 import {
   ApplicationExpression,
   Context,
@@ -221,7 +222,7 @@ const applyRight = (expressions: Expression[]): ApplicationExpression => {
   // [...rest, e1, e2] = expressions; isn't legal TS
   const length = expressions.length;
   if (length < 2) {
-    throw new Error('applyRight requires at least two expressions');
+    throw new InferenceError('applyRight requires at least two expressions');
   }
   const rest = expressions.slice(0, length - 2);
   const [e1, e2] = expressions.slice(length - 2);

src/w.ts

@@ -2,13 +2,14 @@
 // Copyright (c) 2023 Maykin
 //
 // SPDX-License-Identifier: MIT
+import {InferenceError} from './exceptions';
 import {Substitution, generalise, instantiate, makeSubstitution, newTypeVar, unify} from './helper';
 import {Context, Expression, MonoType, PolyType, makeContext} from './models';
 
 export const W = (typEnv: Context, expr: Expression): [Substitution, MonoType] => {
   if (expr.type === 'var') {
     const value = typEnv[expr.x];
-    if (value === undefined) throw new Error(`Undefined variable: ${expr.x}`);
+    if (value === undefined) throw new InferenceError(`Undefined variable: ${expr.x}`);
     return [makeSubstitution({}), instantiate(value)];
   }
 
@@ -78,5 +79,5 @@ export const W = (typEnv: Context, expr: Expression): [Substitution, MonoType] =
     );
     return [s2(s1), t2];
   }
-  throw new Error('Unknown expression type');
+  throw new InferenceError('Unknown expression type');
 };