What Made Lisp Different

paulgraham.com

Technology

3 min read → 55 sec

Lisp differed from Fortran by introducing nine ideas, from conditionals and recursion to macros and runtime compilation. Many later languages adopted the first set of ideas, while Lisp's code-as-tree model remains distinctive.

Lisp introduced nine ideas that reshaped programming.

Conditionals, first-class functions, recursion, and garbage collection spread widely. Tree syntax and read-compile-run unity mostly stayed Lisp's.

McCarthy invented conditionals while developing Lisp.

He then got them into Algol and later languages.

Fortran had only a conditional goto.

It closely modeled hardware branch instructions.

Lisp treated functions as a data type.

Functions had literals, variables, and argument passing.

Lisp was the first programming language to support recursion.

In Lisp, values have types.

Variables effectively hold pointers to values.

Fortran split expressions from statements.

Its punched-card, line-oriented form could not nest statements.

Block-structured languages removed that limit.

Fortran and Algol had already entrenched the expression-statement split.

Lisp made programs fully composable.

Its code trees are expressions that always return values.

When everything is an expression, conditionals and assignments compose in either direction.

Symbols are not strings.

Symbol equality can be tested by comparing a pointer.

Running code at compile-time enables macros.

Compiling at runtime enables extension languages like Emacs.

Reading at runtime lets programs communicate with s-expressions.

That idea was later reinvented as XML.

Expression-only programs are entering the mainstream.

Tree syntax and read-compile-run unity remain largely unique to Lisp.