Nix Repl : add `builtins.*` to global scope

Development
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I would love to have all of the builtins automatically added to my global scope when I’m playing around in Nix Repl. Even a little trick that someone might know so I can add it as a nix repl -E '...' wrapper or something as a shell alias.

I CANNOT be bothered to write builtins. ever again. :joy:

Essentially I want with builtins; or with lib; statements to persist for the life of the REPL. :l <nixpkgs/lib> actually accomplishes this for lib, but it wouldn’t work for actual attributes.

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I’d prefer to have something like Haskell’s .ghci file. You can have one in your working directory or your home directory, and each line in the file is used as a command to ghci when it starts up. So you could just add :a builtins to such a file for nix repl

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Yes!
I might need to spend a weekend to extend it or something. I’m sure its not even a big push. I get the sense that the repl just wasnt developed all that much.

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Well, it sounds like you could just write :a builtins as the first line in the REPL.

In any case, you could do something like nix repl <(echo builtins) and this will have the same effect as running :a builtins as the first line.

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5

I can’t seem to get this to work. I get:

λ nix repl <(echo builtins)                                                       
Welcome to Nix version 2.2. Type :? for help.

Loading '/proc/self/fd/12'...
error: getting status of '/proc/self/fd/pipe:[183479]': No such file or directory
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Huh you’re right, it doesn’t work on NixOS. It works on macOS though. I don’t know what the difference is.

Alternatively you could just create a file with the contents builtins somewhere and then pass that file as an argument to nix repl.

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nix repl =(echo builtins) works. Not too sure what nix tries to do with that file that a pipe does not support.

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=(echo builtins) must be a zsh thing? That’s certainly not supported by Bash. The Fish equivalent is nix repl (echo builtins | psub) which also works on NixOS. I’m not aware of any Bash equivalent for “write the contents to a temporary file, pass it to the command, and delete the file when the job completes”.

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Not meant seriously, but if you don’t want to create a file¹ or use a different shell you could also vastly overengineer this by using a base64-encoded tar.xz passed via file:// URI:

nix repl file://<(echo /Td6WFoAAATm1rRGAgAhARwAAAAQz1jM4Cf/AJFdADEdSVZjYCMVSnMdMhyem4Lxxsmjn4RXoab9vopKoxzV9eZZmo2FmbaJDuB7gNe/VK+gFE5lI3VUCGmKWNs5JxAOhNQCAyaT6ByczZES8wfdfHgKM4oHELnfMJqsA+mNLkgJIDsmut7h6XhJahRCVoqMN/kMT3MWyPOfg830HAcnk/e9xPGvEGeoDy2t1ySKqkAAAAAASMZR5KVyXMgAAa0BgFAAANZnUtqxxGf7AgAAAAAEWVo= | base64 -d)

¹ Well, practically you create files in Nix’s cache and the store :wink:

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Clearly someone needs to submit a PR to teach nix repl to accept a data: URI, thus allowing you to say nix repl data:text/plain,builtins. Or maybe just teach it accept plain text URLs in addition to tar.xz ones so you can say file://<(echo builtins).

Or I guess we could figure out why nix repl <(echo builtins) fails and fix it.

I filed an issue for this nix repl failure. I believe the underlying cause is that nix repl resolves the symlink and tries to read the underlying path, instead of just reading from the symlink directly. File as `nix repl` tries to resolve symlinks as arguments, fails on pipe redirections · Issue #2771 · NixOS/nix · GitHub

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You can use an expect wrapper around nix repl that does what you need after it starts. The following always imports builtins:

#!/usr/bin/env nix-shell
#!nix-shell -i expect -p expect
spawn -noecho nix repl {*}$argv
expect "nix-repl> " {
  send ":a builtins\n"
  interact
}

But you can see that it’s easy to add more commands :slight_smile:

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12

I love this answer.
Wildly over engineered/obfuscated solutions to simple peoblems is kind of my aesthetic :joy:

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