Add support for nested templates

Add support for expressions inside expressions.

Add support for templating where the output of one expression results in
another template part with expressions.

E.g., this did not work before, but compilation will now evaluate both
with expression with `$condition` and parameter substitution with
`$text`:

```
{{ with $condition }}
  echo '{{ $text }}'
{{ end }}
```

Add also more sanity checks (validation logic) when compiling
expressions to reveal problems quickly.

src/application/Parser/Script/Compiler/Expressions/Expression/ExpressionPosition.ts

@@ -13,4 +13,22 @@ export class ExpressionPosition {
       throw Error(`negative start position: ${start}`);
     }
   }
+
+  public isInInsideOf(potentialParent: ExpressionPosition): boolean {
+    if (this.isSame(potentialParent)) {
+      return false;
+    }
+    return potentialParent.start <= this.start
+      && potentialParent.end >= this.end;
+  }
+
+  public isSame(other: ExpressionPosition): boolean {
+    return other.start === this.start
+      && other.end === this.end;
+  }
+
+  public isIntersecting(other: ExpressionPosition): boolean {
+    return (other.start < this.end && other.end > this.start)
+      || (this.end > other.start && other.start >= this.start);
+  }
 }

src/application/Parser/Script/Compiler/Expressions/ExpressionsCompiler.ts

@@ -20,21 +20,59 @@ export class ExpressionsCompiler implements IExpressionsCompiler {
     if (!code) {
       return code;
     }
-    const expressions = this.extractor.findExpressions(code);
-    ensureParamsUsedInCodeHasArgsProvided(expressions, args);
     const context = new ExpressionEvaluationContext(args);
-    const compiledCode = compileExpressions(expressions, code, context);
+    const compiledCode = compileRecursively(code, context, this.extractor);
     return compiledCode;
   }
 }
 
+function compileRecursively(
+  code: string,
+  context: IExpressionEvaluationContext,
+  extractor: IExpressionParser,
+): string {
+  /*
+    Instead of compiling code at once and returning we compile expressions from the code.
+    And recompile expressions from resulting code recursively.
+    This allows using expressions inside expressions blocks. E.g.:
+    ```
+      {{ with $condition }}
+        echo '{{ $text }}'
+      {{ end }}
+    ```
+    Without recursing parameter substitution for '{{ $text }}' is skipped once the outer
+    {{ with $condition }} is rendered.
+    A more optimized alternative to recursion would be to a parse an expression tree
+    instead of linear expression lists.
+  */
+  if (!code) {
+    return code;
+  }
+  const expressions = extractor.findExpressions(code);
+  if (expressions.length === 0) {
+    return code;
+  }
+  const compiledCode = compileExpressions(expressions, code, context);
+  return compileRecursively(compiledCode, context, extractor);
+}
+
 function compileExpressions(
   expressions: readonly IExpression[],
   code: string,
   context: IExpressionEvaluationContext,
 ) {
+  ensureValidExpressions(expressions, code, context);
   let compiledCode = '';
-  const sortedExpressions = expressions
+  const outerExpressions = expressions.filter(
+    (expression) => expressions
+      .filter((otherExpression) => otherExpression !== expression)
+      .every((otherExpression) => !expression.position.isInInsideOf(otherExpression.position)),
+  );
+  /*
+    This logic will only compile outer expressions if there were nested expressions.
+    So the output of this compilation may result in new uncompiled expressions.
+  */
+  const sortedExpressions = outerExpressions
     .slice() // copy the array to not mutate the parameter
     .sort((a, b) => b.position.start - a.position.start);
   let index = 0;
@@ -65,6 +103,43 @@ function extractRequiredParameterNames(
     .filter((name, index, array) => array.indexOf(name) === index); // Remove duplicates
 }
 
+function printList(list: readonly string[]): string {
+  return `"${list.join('", "')}"`;
+}
+
+function ensureValidExpressions(
+  expressions: readonly IExpression[],
+  code: string,
+  context: IExpressionEvaluationContext,
+) {
+  ensureParamsUsedInCodeHasArgsProvided(expressions, context.args);
+  ensureExpressionsDoesNotExtendCodeLength(expressions, code);
+  ensureNoExpressionsAtSamePosition(expressions);
+  ensureNoInvalidIntersections(expressions);
+}
+
+function ensureExpressionsDoesNotExtendCodeLength(
+  expressions: readonly IExpression[],
+  code: string,
+) {
+  const expectedMax = code.length;
+  const expressionsOutOfRange = expressions
+    .filter((expression) => expression.position.end > expectedMax);
+  if (expressionsOutOfRange.length > 0) {
+    throw new Error(`Expressions out of range:\n${JSON.stringify(expressionsOutOfRange)}`);
+  }
+}
+
+function ensureNoExpressionsAtSamePosition(expressions: readonly IExpression[]) {
+  const instructionsAtSamePosition = expressions.filter(
+    (expression) => expressions
+      .filter((other) => expression.position.isSame(other.position)).length > 1,
+  );
+  if (instructionsAtSamePosition.length > 0) {
+    throw new Error(`Instructions at same position:\n${JSON.stringify(instructionsAtSamePosition)}`);
+  }
+}
+
 function ensureParamsUsedInCodeHasArgsProvided(
   expressions: readonly IExpression[],
   providedArgs: IReadOnlyFunctionCallArgumentCollection,
@@ -80,6 +155,16 @@ function ensureParamsUsedInCodeHasArgsProvided(
   }
 }
 
-function printList(list: readonly string[]): string {
-  return `"${list.join('", "')}"`;
+function ensureNoInvalidIntersections(expressions: readonly IExpression[]) {
+  const intersectingInstructions = expressions.filter(
+    (expression) => expressions
+      .filter((other) => expression.position.isIntersecting(other.position))
+      .filter((other) => !expression.position.isSame(other.position))
+      .filter((other) => !expression.position.isInInsideOf(other.position))
+      .filter((other) => !other.position.isInInsideOf(expression.position))
+      .length > 0,
+  );
+  if (intersectingInstructions.length > 0) {
+    throw new Error(`Instructions intersecting unexpectedly:\n${JSON.stringify(intersectingInstructions)}`);
+  }
 }