+++
title = "Check Command"
description = "Validate MLF lexicon files"
weight = 4
+++

The `mlf check` command validates MLF lexicon files for syntax and type errors.

## Usage

```bash
mlf check [INPUT]...
```

**Arguments:**
- `[INPUT]...` - MLF lexicon file(s) to validate (glob patterns supported)

If no input files are provided, `mlf check` will use the source directory from your `mlf.toml` configuration.

## Examples

### Check with Configuration

If you have an `mlf.toml` file:

```toml
[source]
directory = "./lexicons"
```

Simply run:

```bash
mlf check
```

This automatically checks all `.mlf` files in `./lexicons/`.

### Check Specific Files

```bash
# Check a single file
mlf check thread.mlf

# Check multiple files
mlf check thread.mlf profile.mlf reply.mlf
```

### Check with Glob Patterns

```bash
# Check all MLF files in a directory
mlf check "lexicons/**/*.mlf"

# Check files matching a pattern
mlf check "src/*/schema.mlf"

# Check all MLF files recursively
mlf check "**/*.mlf"
```

## Validation Checks

The check command performs comprehensive validation:

### 1. **Syntax Validation**
- Correct MLF syntax
- Proper use of keywords
- Valid identifiers and namespaces
- Correct constraint syntax

### 2. **Type Validation**
- All referenced types exist
- Type constraints are valid
- Required fields are properly marked
- Array and object structures are correct

### 3. **Semantic Validation**
- No duplicate definitions
- Valid record keys
- Proper XRPC method signatures
- Correct union type usage

### 4. **Cross-Reference Validation**
- External references resolve correctly
- Import statements are valid
- Prelude types are accessible
- Fetched dependencies are available

## Output

### Success

When all files are valid:

```
✓ thread.mlf: Parsed successfully
✓ profile.mlf: Parsed successfully
✓ reply.mlf: Parsed successfully

✓ All lexicons are valid
```

### Errors

When errors are found, detailed diagnostics are shown:

```
  × Undefined reference to 'ProfileView'
   ╭─[profile.mlf:5:12]
 5 │     author: ProfileView,
   ·             ^^^^^^^^^^^ 'ProfileView' is not defined
   ╰────
  help: Make sure this type is defined in the same file or imported via 'use'.
```

Each error includes:
- Error message with context
- Source code snippet
- Exact location (line and column)
- Helpful suggestions for fixing

## Working with Dependencies

If your lexicons reference external types (e.g., from `app.bsky` or `com.atproto`), make sure to fetch them first:

```bash
# Fetch dependencies
mlf fetch

# Then check your lexicons
mlf check
```

The check command automatically loads lexicons from `.mlf/lexicons/mlf/` if they exist.

## Exit Codes

- `0` - All files are valid
- `1` - Validation errors found

## Common Issues

### Undefined Reference

```
× Undefined reference to 'SomeType'
```

**Solution:**
- Define `SomeType` in the same file, or
- Fetch the lexicon containing `SomeType` using `mlf fetch`, or
- Add an import statement if needed

### Parse Error

```
× Expected 'constrained', found 'contsrained'
```

**Solution:** Fix the typo in your MLF file

### Type Mismatch

```
× Field 'count' expects integer, found string
```

**Solution:** Correct the field type to match the schema

## Tips

1. **Use configuration** - Set up `mlf.toml` to avoid typing paths repeatedly
2. **Check often** - Run `mlf check` frequently during development
3. **Version control** - Commit valid lexicons only
4. **Fetch first** - Always fetch dependencies before checking
5. **Read errors carefully** - MLF provides detailed error messages with helpful suggestions