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John Carmack on mutable variables

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498 points azhenley | 5 comments | 31 Oct 25 02:34 UTC | HN request time: 0s | source
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munchler ◴[31 Oct 25 02:54 UTC] No.45767832[source]▶
> Making almost every variable const at initialization is good practice. I wish it was the default, and mutable was a keyword.

It's funny how functional programming is slowly becoming the best practice for modern code (pure functions, no side-effects), yet functional programming languages are still considered fringe tech for some reason.

If you want a language where const is the default and mutable is a keyword, try F# for starters. I switched and never looked back.

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skeezyjefferson ◴[31 Oct 25 12:09 UTC] No.45771134[source]▶
> If you want a language where const is the default and mutable is a keyword

whats the difference between const and mutable?

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1. Thorrez ◴[31 Oct 25 12:15 UTC] No.45771189[source]▶
"const" means something can't be changed. "mutable" means it can be changed.

You don't need both a "const" keyword and a "mutable" keyword in a programming language. You only need 1 of the keywords, because the other can be the default. munchler is saying the "const" keyword shouldn't exist, and instead all variables should be constant by default, and we should have a "mutable" keyword to mark variables as mutable.

As opposed to how C++ works today, where there is no "mutable" keyword, variables are mutable by default, and we use a "const" keyword to mark them constant.

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2. alcover ◴[31 Oct 25 13:07 UTC] No.45771571[source]▶
> you don't need both a "const" keyword and a "mutable" keyword

What if the lang has pointers? How express read-only?

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3. skeezyjefferson ◴[31 Oct 25 15:28 UTC] No.45773144[source]▶
compiler flags with line and column number seems like the easiest way
4. Thorrez ◴[01 Nov 25 13:09 UTC] No.45781369[source]▶
You can make everything read-only by default, and if you need non-read-only, you use "mutable".
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5. alcover ◴[01 Nov 25 19:16 UTC] No.45784475{3}[source]▶
You need two keywords. One for assignability and one for writability :

    const ptr;  // can't reassign, can't write-through (if r/o by default)
    const mut ptr;  // can write