docs(examples): add basic validation example

Demonstrates core Veritext functionality: metrics, validators, composites,
and constraint validators with runnable code.

examples/basic_validation.py

@@ -0,0 +1,135 @@
+"""Basic text validation examples.
+
+Demonstrates core Veritext functionality:
+- Single metric scoring (BLEU, ROUGE)
+- Validator usage with thresholds
+- Composite validators (all_of, any_of)
+- Constraint validators (length, readability)
+"""
+
+from veritext.core.types import ValidationContext
+from veritext.metrics import Bleu, Rouge
+from veritext.validators import (
+    all_of,
+    any_of,
+    bleu,
+    contains,
+    excludes,
+    length,
+    readability,
+    rouge,
+)
+
+
+def metric_scoring_example() -> None:
+    """Score text using individual metrics."""
+    candidate = "The quick brown fox jumps over the lazy dog."
+    reference = "A fast brown fox leaps over a sleepy dog."
+
+    # BLEU scoring (translation quality)
+    bleu_metric = Bleu()
+    bleu_result = bleu_metric.score(candidate, reference)
+    print("BLEU Scores:")
+    print(f"  BLEU-1: {bleu_result.bleu1:.3f}")
+    print(f"  BLEU-4: {bleu_result.bleu4:.3f}")
+    print(f"  Brevity penalty: {bleu_result.brevity_penalty:.3f}")
+
+    # ROUGE scoring (summary quality)
+    rouge_metric = Rouge()
+    rouge_result = rouge_metric.score(candidate, reference)
+    print("\nROUGE Scores:")
+    print(f"  ROUGE-1 F1: {rouge_result.rouge1.fmeasure:.3f}")
+    print(f"  ROUGE-L F1: {rouge_result.rouge_l.fmeasure:.3f}")
+
+
+def validator_example() -> None:
+    """Use validators to make pass/fail decisions."""
+    reference = "Machine learning models require training data."
+    candidate = "ML models need training data to learn patterns."
+
+    context = ValidationContext(reference=reference)
+
+    # BLEU validator with minimum threshold
+    bleu_validator = bleu(min_score=0.3)
+    result = bleu_validator.check(candidate, context)
+    print(f"\nBLEU validation (min 0.3): {'PASS' if result.passed else 'FAIL'}")
+
+    # ROUGE validator
+    rouge_validator = rouge(min_score=0.5)
+    result = rouge_validator.check(candidate, context)
+    print(f"ROUGE validation (min 0.5): {'PASS' if result.passed else 'FAIL'}")
+
+
+def composite_validator_example() -> None:
+    """Combine validators with all_of and any_of."""
+    reference = "The product launch exceeded all expectations."
+    candidate = "The product release performed beyond expectations."
+
+    context = ValidationContext(reference=reference)
+
+    # All checks must pass
+    strict_validator = all_of(
+        [
+            bleu(min_score=0.2),
+            rouge(min_score=0.4),
+            length(max_chars=100),
+        ]
+    )
+    result = strict_validator.check(candidate, context)
+    print(f"\nStrict (all_of): {'PASS' if result.passed else 'FAIL'}")
+    if not result.passed:
+        print(f"  Failures: {result.failure_summary}")
+
+    # At least one check must pass
+    flexible_validator = any_of(
+        [
+            bleu(min_score=0.8),  # Unlikely to pass
+            rouge(min_score=0.4),  # More likely
+        ]
+    )
+    result = flexible_validator.check(candidate, context)
+    print(f"Flexible (any_of): {'PASS' if result.passed else 'FAIL'}")
+
+
+def constraint_validator_example() -> None:
+    """Use constraint validators for text properties."""
+    text = "This short guide explains the basics clearly."
+    context = ValidationContext()  # No reference needed for constraints
+
+    # Length constraints
+    length_validator = length(min_chars=20, max_chars=100, min_words=5, max_words=20)
+    result = length_validator.check(text, context)
+    print(f"\nLength check: {'PASS' if result.passed else 'FAIL'}")
+
+    # Readability (Flesch-Kincaid)
+    readability_validator = readability(max_grade=10.0)
+    result = readability_validator.check(text, context)
+    print(f"Readability (grade <= 10): {'PASS' if result.passed else 'FAIL'}")
+
+    # Content patterns
+    contains_validator = contains(patterns=["guide", "basics"])
+    result = contains_validator.check(text, context)
+    print(f"Contains required terms: {'PASS' if result.passed else 'FAIL'}")
+
+    excludes_validator = excludes(patterns=["error", "warning"])
+    result = excludes_validator.check(text, context)
+    print(f"Excludes forbidden terms: {'PASS' if result.passed else 'FAIL'}")
+
+
+def main() -> None:
+    """Run all examples."""
+    print("=" * 60)
+    print("Veritext Basic Validation Examples")
+    print("=" * 60)
+
+    metric_scoring_example()
+    validator_example()
+    composite_validator_example()
+    constraint_validator_example()
+
+    print("\n" + "=" * 60)
+    print("All examples completed.")
+
+
+if __name__ == "__main__":
+    main()