Recursion in Nix language - did the language change or am I mistyping the factorial example?

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1

Hi Nix Discourse Help,

I am reading this tutorial from 2014 about the Nix language:

I add the following to a file myfactorial.nix

let
  # factorial : int -> int
  factorial = n: if n == 0 then 1
                           else n * factorial (n-1);
in
  factorial 9

and evaluate with:

nix-instantiate --eval myfactorial.nix

and get back:

error: undefined variable 'n-1'

       at /home/i97henka/github/hk.nix.dev.walkthrough/myfactorial.nix:4:48:

            3|   factorial = n: if n == 0 then 1
            4|                            else n * factorial (n-1);
             |                                                ^
            5| in

I have tried to double check my work to make sure that I didn’t accidentally mistype anything.
Please let me know what gives.

$ nix --version
nix (Nix) 2.11.1
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2

I guess you meant n - 1. Identifiers can have dashes in it, so n-1 is (mis)interpreted as identifier here.

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3

Thank you, that solved my problem :pray:. The example was slightly misformatted in that case.

4

Another blog bug squashed. However its very hard to update blog posts and such to reflect fixes. Not everything is tracked and deployed by git :-(((((

There is no way around this problem.

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5

I guess it was just a typo (or just muscle memory, I mean in almost every other language n-1 is a subtraction).

When I copy the strings from the blogpost I get (n — 1), so everything seems fine. I mean, if the Nix language would suddenly change its behavior in such a drastic way that would be very bad :wink:

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