为何Module#define_method会无视Ruby方法命名规则？

Great question! Let's break this down clearly:

The core difference: Syntax parsing vs runtime API #

When you use the def keyword, you're working within Ruby's syntax parsing stage. The Ruby parser enforces strict rules for method names here—they have to be valid identifiers (start with a letter/underscore, followed by letters, numbers, or underscores; no special characters like . allowed). That's why this throws a syntax error immediately:

def the.dot :dot end
# Syntax error, unexpected '.'

The parser sees the as a variable, then .dot as an attempt to call a dot method on it—it never recognizes the.dot as a single method name.

On the other hand, define_method is a runtime API. It skips the syntax parsing checks entirely, and directly modifies the method table of the current class/module. Ruby's method tables are essentially hash maps where keys are symbols—and Ruby lets you create symbols from almost any string (even those with dots, spaces, or other special characters).

Example walkthrough #

When you run this code:

define_method('the.dot') { :dot } #=> :"the.dot"
method('the.dot') #=> #<Method: main.the.dot>

You're telling Ruby to add a new entry to the method table: the symbol :"the.dot" maps to your block. No syntax parser is involved here—you're just manipulating the runtime state directly.

Why regular calls fail, but runtime dispatch works #

When you try the.dot, you're back to the syntax parser. It interprets the as an object and .dot as a call to a dot method on that object—hence the NameError because the isn't defined.

But methods like public_send or method(name).call are runtime calls that take a symbol/string as the method name. They bypass the syntax parser and look directly in the method table for the matching symbol:

method('the.dot').call #=> :dot
public_send('the.dot') #=> :dot

Quick summary #

• def is bound by Ruby's syntax rules because it's processed during parsing.
• define_method operates at runtime, directly interacting with Ruby's method table, which allows any valid symbol (derived from almost any string) as a method name.
• Calling these "unconventional" methods requires runtime dispatch (like send) since the regular dot-syntax can't parse their names.

内容的提问来源于stack exchange，提问作者Andrey Deineko