> ## Documentation Index
> Fetch the complete documentation index at: https://dsrs.herumbshandilya.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Constraints

> Validate LM outputs with check and assert expressions

Constraints let you validate what the LM returns. Add them to output fields to enforce rules like "confidence must be between 0 and 1" or "answer can't be empty".

## Two Flavors

**`#[check]`** - Soft constraint. The call succeeds, but you can see if the check passed or failed.

**`#[assert]`** - Hard constraint. The call fails if the assertion doesn't hold.

## Check Example

```rust theme={null}
#[derive(Signature, Clone, Debug)]
struct QA {
    #[input]
    question: String,

    #[output]
    answer: String,

    #[output]
    #[check("this >= 0.0 and this <= 1.0", label = "valid_range")]
    confidence: f32,
}
```

If the LM returns `confidence: 1.5`, the call still succeeds - you just see `passed: false` when you inspect the check result.

The `label` is required for checks. It's how you identify which constraint passed or failed.

## Assert Example

```rust theme={null}
#[derive(Signature, Clone, Debug)]
struct StrictQA {
    #[input]
    question: String,

    #[output]
    #[assert("this.len() > 0")]
    answer: String,

    #[output]
    #[assert("this >= 0.0 and this <= 1.0", label = "confidence_range")]
    confidence: f32,
}
```

If confidence is 1.5, the call returns a `PredictError::Parse` error. Label is optional for asserts.

## Writing Expressions

Use `this` to refer to the field value:

```python theme={null}
this > 0
this >= 0.0 and this <= 1.0
this == "expected"
this != "bad"
```

Boolean logic:

```python theme={null}
this > 0 and this < 100
this == "a" or this == "b"
not this
```

String methods:

```python theme={null}
this.len() > 0
this.startswith("https://")
this.endswith(".com")
this.contains("@")
```

Collections:

```python theme={null}
this.len() < 10      # list has fewer than 10 items
this[0] == "first"   # first item equals "first"
```

## Inspecting Check Results

`Predicted<O>` carries per-field metadata including constraint results. Access it via `.metadata()`:

```rust theme={null}
let predict = Predict::<QA>::new();
let result = predict.call(QAInput {
    question: "What is the capital of France?".into(),
}).await?;

// Output fields are available directly via Deref
println!("Answer: {}", result.answer);

// Inspect per-field constraint results via metadata
if let Some(field_meta) = result.metadata().field_meta.get("confidence") {
    for check in &field_meta.checks {
        if !check.passed {
            println!("Check '{}' failed", check.label);
        }
    }
}
```

## Handling Assert Failures

When an assert fails, you get a `PredictError`:

```rust theme={null}
match predict.call(input).await {
    Ok(result) => {
        println!("{}", result.answer);
    }
    Err(PredictError::Parse { source, .. }) => {
        // The LM returned something that violated an assertion
        eprintln!("Invalid response: {source}");
    }
    Err(e) => {
        eprintln!("Other error: {e}");
    }
}
```

## Common Patterns

**Probability/confidence range:**

```rust theme={null}
#[check("this >= 0.0 and this <= 1.0", label = "valid_probability")]
confidence: f64,
```

**Non-empty output:**

```rust theme={null}
#[assert("this.len() > 0")]
answer: String,
```

**Length limits:**

```rust theme={null}
#[check("this.len() < 500", label = "max_length")]
summary: String,
```

**URL format:**

```rust theme={null}
#[assert("this.startswith('https://')")]
url: String,
```

**Multiple constraints on one field:**

```rust theme={null}
#[output]
#[check("this.len() > 0", label = "non_empty")]
#[check("this.len() < 500", label = "not_too_long")]
response: String,
```

## Compile-time Errors

If you forget the label on a check:

```rust theme={null}
#[check("this > 0")]  // oops, no label
score: i32,
```

You get:

```
error: #[check] requires a label: #[check("expr", label = "name")]
 --> src/lib.rs:9:5
  |
9 |     #[check("this > 0")]
  |     ^^^^^^^^^^^^^^^^^^^^
```