[Feature] Provide better message when using getProp

I have a checker and using it to check some of it props

const propertyName: string = nameof<MyModel>((o) => o.headlines);
// propertyName = 'headlines'
// value is an array / should be [string, string, string?]
MyModelChecker.getProp(propertyName).strictCheck(value);

I provoked an error and the result was

Error: value[0] is not a string
    at new VError (util.js?5394:20)
    at DetailContext.getError (util.js?5394:91)
    at Checker._doCheck (index.js?7160:179)
    at Checker.strictCheck (index.js?7160:101)
    at validator (MyComponent.ts?b499:24)
    at assertProp (vue.esm.js?a026:1722)
    at validateProp (vue.esm.js?a026:1641)
    at loop (vue.esm.js?a026:4668)
    at initProps (vue.esm.js?a026:4701)
    at initState (vue.esm.js?a026:4642)

Could we provide the parameter value of getProp in the message, so I could see which property is checked?

The message could be headlines[0] is not a string or it could be MyModelChecker's headlines found: value[0] is not a string or similar

I’m open to discussion and I’m willing to implement this feature once we’ve found a good design

[dsagal]

I added setReportedPath() method. E.g.

MyModelChecker.setReportedPath('MyModel');
MyModelChecker.getProp(propertyName).strictCheck(value);

Should report, on error, MyModel.headlines[0] is not a string. I released a patch version 0.1.9.

[dsagal]

This is not unreasonable, in fact, including a diff below which might be what you are after. On the other hand, if the caller is using getProp, then the property is name available, so perhaps it’s OK (better?) to leave it to the caller to format errors appropriately.

E.g. in your case, you could add a helper:

function getCheckerFunc(rootChecker: Checker, propertyName: string) {
  const checker = rootChecker.getProp(propertyName);
  return (value: any) => {
    try {
      checker.strictCheck(value);
    } catch (e) {
      e.message = `Property '${propertyName}': ${e.message}`;
      throw e;
    }
  };
}

Diff of possible implementation changing getProp() to throw errors of the form propName is not a ...:

diff --git a/lib/index.ts b/lib/index.ts
index 46490b3..b59d106 100644
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -43,7 +43,7 @@ export class Checker {
   private checkerStrict: CheckerFunc;

   // Create checkers by using `createCheckers()` function.
-  constructor(private suite: ITypeSuite, private ttype: TType) {
+  constructor(private suite: ITypeSuite, private ttype: TType, private _path?: string) {
     if (ttype instanceof TIface) {
       for (const p of ttype.props) {
         this.props.set(p.name, p.ttype);
@@ -104,7 +104,7 @@ export class Checker {
   public getProp(prop: string): Checker {
     const ttype = this.props.get(prop);
     if (!ttype) { throw new Error(`Type has no property ${prop}`); }
-    return new Checker(this.suite, ttype);
+    return new Checker(this.suite, ttype, prop);
   }

   /**
@@ -160,7 +160,7 @@ export class Checker {
     if (!checkerFunc(value, noopCtx)) {
       const detailCtx = new DetailContext();
       checkerFunc(value, detailCtx);
-      throw detailCtx.getError();
+      throw detailCtx.getError(this._path);
     }
   }

@@ -171,7 +171,7 @@ export class Checker {
     }
     const detailCtx = new DetailContext();
     checkerFunc(value, detailCtx);
-    return detailCtx.getErrorDetail();
+    return detailCtx.getErrorDetail(this._path);
   }

   private _getMethod(methodName: string): TFunc {
diff --git a/lib/util.ts b/lib/util.ts
index d909f93..cab6a83 100644
--- a/lib/util.ts
+++ b/lib/util.ts
@@ -83,8 +83,7 @@ export class DetailContext implements IContext {
     }
   }

-  public getError(): VError {
-    let path: string = "value";
+  public getError(path: string = "value"): VError {
     const msgParts: string[] = [];
     for (let i = this._propNames.length - 1; i >= 0; i--) {
       const p = this._propNames[i];
@@ -97,8 +96,7 @@ export class DetailContext implements IContext {
     return new VError(path, msgParts.join("; "));
   }

-  public getErrorDetail(): IErrorDetail|null {
-    let path: string = "value";
+  public getErrorDetail(path: string = "value"): IErrorDetail|null {
     const details: IErrorDetail[] = [];
     for (let i = this._propNames.length - 1; i >= 0; i--) {
       const p = this._propNames[i];