Talk:M-expression

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It is perhaps notable that the computer algebra system Mathematica uses the M-Expression syntax as its "generic" representation of data and functions. —Preceding unsigned comment added by JasticE (talk • contribs) 22:08, 17 November 2004

This page doesn't say what an M-expression is. — Preceding unsigned comment added by 81.164.106.200 (talk) 18:42, 14 February 2013 (UTC)Reply

Given that Lisp encoded as S-expressions is homoiconic, and that M-expressions form a one-to-one, invertible correspondence with their equivalent S-expressions, it should then be obvious that M-expressions are thus also homoiconic. Therefore, no expressive power is lost by the utilization of M-expression syntax compared to S-expression syntax.

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in the table example...

[lessp[x;0] → minus[x]; T → x] (cond ((< x 0) (- x)) (t x))

... why e.g. "(t x)" instead of just 'x' or "(x)"?

(I'm a noob re Lisp.) Abe149 (talk) 04:02, 26 July 2023 (UTC)Reply

COND clauses starts with a condition. Anything that isn't NIL is true, but the symbol T is used as the canonical value for truth. So `(t x)` is a clause that is always selected if we reach it (because its condition is just T) and whose result is the value of X.

X by itself is a syntax error (at least in Common Lisp and Scheme) because COND clauses need to be lists. `(X)` would work here: X is used as the condition, and if it's true, its value will be returned since we don't give an explicit value for that clause, and if X is NIL, then we run out of COND clauses, so the result of the COND will be NIL. The difference is that `(X)` is selected or not based off whether X is NIL, and if it's not selected, we only get NIL because no clauses were selected; if you put another clause after `(t x)`, it would never be selected (because T is always true), but if you put one after `(x)`, it would be selected in the cases where X is NIL.

`(t x)` also matches the mexpr syntax which needs something on the left of the arrow. 204.83.110.176 (talk) 16:19, 30 December 2023 (UTC)Reply