lix/lix/nix/repl.md

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Note: this command's interface is based heavily around installables, which you may want to read about first (nix --help).

Examples

• Display all special commands within the REPL:

  # nix repl
  nix-repl> :?
  

• Evaluate some simple Nix expressions:

  # nix repl
  
  nix-repl> 1 + 2
  3
  
  nix-repl> map (x: x * 2) [1 2 3]
  [ 2 4 6 ]
  

• Interact with Nixpkgs in the REPL:

  # nix repl --file example.nix
  Loading Installable ''...
  Added 3 variables.
  
  # nix repl --expr '{a={b=3;c=4;};}'
  Loading Installable ''...
  Added 1 variables.
  
  # nix repl --expr '{a={b=3;c=4;};}' a
  Loading Installable ''...
  Added 1 variables.
  
  # nix repl --extra-experimental-features 'flakes' nixpkgs
  Loading Installable 'flake:nixpkgs#'...
  Added 5 variables.
  
  nix-repl> legacyPackages.x86_64-linux.emacs.name
  "emacs-27.1"
  
  nix-repl> legacyPackages.x86_64-linux.emacs.name
  "emacs-27.1"
  
  nix-repl> :q
  
  # nix repl --expr 'import <nixpkgs>{}'
  
  Loading Installable ''...
  Added 12439 variables.
  
  nix-repl> emacs.name
  "emacs-27.1"
  
  nix-repl> emacs.drvPath
  "/nix/store/lp0sjrhgg03y2n0l10n70rg0k7hhyz0l-emacs-27.1.drv"
  
  nix-repl> drv = runCommand "hello" { buildInputs = [ hello ]; } "hello; hello > $out"
  
  nix-repl> :b drv
  this derivation produced the following outputs:
    out -> /nix/store/0njwbgwmkwls0w5dv9mpc1pq5fj39q0l-hello
  
  nix-repl> builtins.readFile drv
  "Hello, world!\n"
  
  nix-repl> :log drv
  Hello, world!
  

Description

This command provides an interactive environment for evaluating Nix expressions. (REPL stands for 'read–eval–print loop'.)

On startup, it loads the Nix expressions named files and adds them into the lexical scope. You can load addition files using the :l <filename> command, or reload all files using :r.

Adding default variables in REPL sessions

It is possible to automatically load variables from a list of files into each new REPL session using the repl-overlays configuration option.

Each file should contain a Nix function taking three taking and returning an attribute set. These three arguments are:

1. An attribute set containing at least a currentSystem attribute (this is identical to builtins.currentSystem, except that it's available in pure-eval mode).
2. The final top-level bindings produced by calling all repl-overlays.
3. The top-level bindings produced by the previous repl-overlays value (or the default top-level bindings).

Examples

• Aliasing legacyPackages.${currentSystem} to pkgs

  A file named /home/alice/my-overlays.nix containing the following code would, if legacyPackages exists, add a variable named pkgs into the global REPL scope, which returns the value of legacyPackages.${currentSystem}.

  info: final: prev:
  if prev ? legacyPackages
    && prev.legacyPackages ? ${info.currentSystem}
  then
  {
    pkgs = prev.legacyPackages.${info.currentSystem};
  }
  else
  { }
  

  This file can be loaded automatically for every REPL session by adding it to the value of repl-overlays inside nix.conf. For a single session, it is possible to add it using --option repl-overlays /home/alice/my-overlay.nix:

  # nix repl --option repl-overlays /home/alice/my-overlay.nix nixpkgs
  
  nix-repl> pkgs == legacyPackages.${builtins.currentSystem}
  true
  
  nix-repl> pkgs.hello
  «derivation /nix/store/qdzln99hynf92vrz8sz91hlf1dmb1vdy-hello-2.12.2.drv»
  

See the nix.conf repl-overlays documentation for more information.

)""