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3-LISP WORKING GUIDE#13: Patterns

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Issue #13:Parameter patterns

Description:Exactly how powerful will the pattern matcher be? Will it spread rails as well as sequences? What about &OPTIONAL and &REST features? What’s an error as opposed to a failure? How does pattern matching fit in with LET expressions?

Status:Unresolved

Last Edited:September 20, 1982 (Jim des Rivières)

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1> ((lambda simple x x) . [1 2 3]); this is 3-LISP
=> [1 2 3]
1> ((lambda simple [x] x) . [1])
=> 1
1> ((lambda simple [x] x) . ’[1]); ’[1] treated like [’1]
=> ’1

The last example, which shows a rail pattern matching a handle, illustrates a handy (nay, essential) feature when defining macros. Unfortunately, it has an ugly side-effect:

1> (type . ’[a]); This isn’t in error!
=> ’atom
1> (type . [’a]); Treated like preceding expression
=> ’atom