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So.... (if (/= den 0.0) (/ num den)) Languages keep reinventing Lisp. Greenspun's 10th rule.

by SeanLuke 4y ago

valenterry 4y ago
Was Lisp really the first language where you could provide a custom if-expression or have a builtin if-expression?
- bheadmaster 4y ago
  As far as I know, Lisp was the first expression-oriented language, so the if-as-an-expression idea probably comes from there. In contrast to Fortran, in which "if" was a strict statement, which doesn't evaluate to anything and cannot be used directly in assignment.
- lispm 4y ago
  FORTRAN I and II then had only an Arithmetic IF statement with three labels to jump to, based on an arithmetic value ( https://en.wikipedia.org/wiki/Arithmetic_IF https://en.wikipedia.org/wiki/Arithmetic_IF ). FORTRAN example: IF ( N ) 10, 20, 30 Above means: if N is less than zero, then jump to label 10 if N is zero, then jump to label 20 if N is greater than zero, then jump to label 30 LISP had a COND expression, with multiple condition clauses. An example with three clauses. (COND ((= x 1) 'one) ((= x 2) 'two) (T 'not-one-or-two)) if X is 1, then return ONE if X is 2, then return TWO ELSE return NOT-ONE-OR-TWO John McCarthy mentions conditional expressions in this paper from 1958: https://www.softwarepreservation.org/projects/LISP/MIT/AIM-001.pdf https://www.softwarepreservation.org/projects/LISP/MIT/AIM-0... In external LISP syntax a COND would have looked similar to this: (x>1 -> 1, x=0 -> 0, x<1 -> -1) Above would mean: if X greater than 1, then return 1 if X is 0, then return 0 if X is smaller than 1, then return -1
haileys 4y ago
Would this not just return nil for the else case? This is already the behaviour of Ruby and Lua as well, of course both are unityped languages where "just return nil" semantics are the natural conclusion. The OP however is discussing how this kind of thing would work in a language with a richer type system, like Rust.
- SeanLuke 4y ago
  I guess the Lisp example is an example of a nullable type, which he's railing against; he'd rather have prewrapped types like Some(result) vs None. But it's still an optional expression, and I'm not convinced that declaring an optional type, doing the expression(s), then unwrapping and checking the result for None, is any better than just using an exception handler. He insists that nullable types force you to check for null, but they don't, at least in his math example here. Just let the runtime do it and throw an exception.
  - f_devd 4y ago
    > Just let the runtime do it and throw an exception. Afaik there are no exceptions in rust only panics & `Result`s (and `Option` I suppose)
- eyelidlessness 4y ago
  It will typically return nil, but it’s worth noting that in some lisps nil is the same as the empty list (). Not quite the same as Option/None, but conceptually more similar than most other languages’ null types.
Sharlin 4y ago
That’s a bit like saying that all languages with an imperative for loop keep reinventing FORTRAN. Although I give you that many of Lisp’s good ideas were unfashionable for a while, reusing those ideas isn’t in itself an “ad hoc” implementation of Lisp.

So.... (if (/= den 0.0) (/ num den)) Languages keep reinventing Lisp. Greenspun's 10th rule.

valenterry 4y ago

Was Lisp really the first language where you could provide a custom if-expression or have a builtin if-expression?

bheadmaster 4y ago
As far as I know, Lisp was the first expression-oriented language, so the if-as-an-expression idea probably comes from there. In contrast to Fortran, in which "if" was a strict statement, which doesn't evaluate to anything and cannot be used directly in assignment.
lispm 4y ago
FORTRAN I and II then had only an Arithmetic IF statement with three labels to jump to, based on an arithmetic value ( https://en.wikipedia.org/wiki/Arithmetic_IF https://en.wikipedia.org/wiki/Arithmetic_IF ). FORTRAN example: IF ( N ) 10, 20, 30 Above means: if N is less than zero, then jump to label 10 if N is zero, then jump to label 20 if N is greater than zero, then jump to label 30 LISP had a COND expression, with multiple condition clauses. An example with three clauses. (COND ((= x 1) 'one) ((= x 2) 'two) (T 'not-one-or-two)) if X is 1, then return ONE if X is 2, then return TWO ELSE return NOT-ONE-OR-TWO John McCarthy mentions conditional expressions in this paper from 1958: https://www.softwarepreservation.org/projects/LISP/MIT/AIM-001.pdf https://www.softwarepreservation.org/projects/LISP/MIT/AIM-0... In external LISP syntax a COND would have looked similar to this: (x>1 -> 1, x=0 -> 0, x<1 -> -1) Above would mean: if X greater than 1, then return 1 if X is 0, then return 0 if X is smaller than 1, then return -1

haileys 4y ago

Would this not just return nil for the else case? This is already the behaviour of Ruby and Lua as well, of course both are unityped languages where "just return nil" semantics are the natural conclusion. The OP however is discussing how this kind of thing would work in a language with a richer type system, like Rust.

SeanLuke 4y ago
I guess the Lisp example is an example of a nullable type, which he's railing against; he'd rather have prewrapped types like Some(result) vs None. But it's still an optional expression, and I'm not convinced that declaring an optional type, doing the expression(s), then unwrapping and checking the result for None, is any better than just using an exception handler. He insists that nullable types force you to check for null, but they don't, at least in his math example here. Just let the runtime do it and throw an exception.
- f_devd 4y ago
  > Just let the runtime do it and throw an exception. Afaik there are no exceptions in rust only panics & `Result`s (and `Option` I suppose)
eyelidlessness 4y ago
It will typically return nil, but it’s worth noting that in some lisps nil is the same as the empty list (). Not quite the same as Option/None, but conceptually more similar than most other languages’ null types.

Sharlin 4y ago

That’s a bit like saying that all languages with an imperative for loop keep reinventing FORTRAN. Although I give you that many of Lisp’s good ideas were unfashionable for a while, reusing those ideas isn’t in itself an “ad hoc” implementation of Lisp.