Language Detection¶

run can automatically detect the programming language from code patterns or file extensions.

How It Works¶

Detection happens in this order:

1. Explicit --lang flag (highest priority)
2. File extension (for file paths)
3. Code pattern matching (for inline code)
4. Default fallback (Python)

File Extension Detection¶

File extensions automatically determine the language:

run script.py    # Python
run main.go      # Go
run app.js       # JavaScript
run hello.rs     # Rust

No need to specify --lang!

Pattern Matching¶

Distinctive syntax is auto-detected:

Rust¶

# Unique fn main() syntax
run "fn main() { println!(\"Hello\"); }"

Go¶

# package main declaration
run "package main; import \"fmt\"; func main() { fmt.Println(\"Hello\") }"

JavaScript¶

# console.log
run "console.log('Hello')"

# Node.js require
run "const fs = require('fs'); console.log('Hello')"

C/C++¶

# #include directive
run "#include <stdio.h>\nint main() { printf(\"Hello\\n\"); }"

Ambiguous Cases¶

Some syntax looks similar across languages:

#  Could be Python, Ruby, Lua, or Perl
run "print('hello')"

Solution: Use --lang explicitly:

#  Explicit
run --lang python "print('hello')"
run --lang ruby "print('hello')"
run --lang lua "print('hello')"

Detection Confidence¶

Best Practices¶

When to Use Auto-Detection¶

• File execution (extension available)
• Distinctive syntax
• Quick one-off commands
• Interactive exploration

When to Use --lang¶

• Ambiguous syntax
• Scripts and automation
• CI/CD pipelines
• When correctness is critical

Checking Detection¶

Test what language is detected:

# REPL mode
$ run
>>> :detect on
>>> print('hello')  # See what's detected

Next Steps¶

Command Reference → REPL Mode →