Add the builtin hashTo

[nlewo]

I would like to add a builtin to convert a hash from one representation to another. Several Nix sub-commands (to-base16, to-sri, ...) already exist to do such conversion but I would like to do that at evaluation time.

Spec

builtins.hashTo <type> <hash-string>
where

• <type> is the expected representation. <type> could be one of the hash representations supported by Nix, which are currently base16, base32, base64 and sri.
• <hash-string> is a string representing a hash. The format of this hash can be inferred.

It would return the hash to the expected representation or an error if the input string doesn't represent a hash.

What do you think?

[zimbatm]

@nlewo what is the motivation of doing the transformation at evaluation time?

[veprbl]

@zimbatm For example, I needed it for NixOS/nixpkgs#53787

[nlewo]

Yep, this is in the same kind of contexts.
@zimbatm I want to generate a list of sources with their hashes which could be consumed by an external entity. I then need to export our hash to a more classical format/representation (our base32 hash representation is home made).

[stale]

I marked this as stale due to inactivity. → More info

[L-as]

This would still be useful.

[tomprince]

It looks like fetchTree requires SRI format hashes, so if niv were to switch to using it, when it is available, it would need something like hashTo, if it wants to support existing configuration that uses a different format.

[stale]

I marked this as stale due to inactivity. → More info

[ShamrockLee]

It's still important to me.

BTW, is it possible to implement it inside the nixpkgs/lib first?

[Atemu]

Theoretically, yes. Practically, writing parsers is absolutely not what the Nix language is tailored to.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/the-length-of-integer-in-the-nix-language/24958/1

[ShamrockLee]

Thu correct synopsis would be

builtins.hashTo algo base hash

so that the hash algorithm would be available to the function. It is similar to the --type *algo* flag in the command line interface.

We could define an additional sriHashTo base hash as builtin or as a wrapper of builtins.hashTo, as the hash algorithm is prefixed in the SRI hash.

[amarshall]

Now that #7708 is merged, I believe this is resolved and can be closed.