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It is: using Flakes you're thrusted into a more pure evaluation mode by default, and it creates a (standard) artifact of the revision you're on: the flake.lock.

You can get almost all the benefits Flakes brings without Flakes using alternatives like niv.

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> It is: using Flakes you're thrusted into a more pure evaluation mode by default, and it creates a (standard) artifact of the revision you're on: the flake.lock.

Can you name a specific example of the kind of nondeterminism/nonreproducibility I risk by simply pinning Nixpkgs itself (or to be more principled, using Niv)?

Certain environment variables at the time "nix-build" is executed can affect the nix configuration. Nix flakes default to ignoring those environment variables.

Flakes are also the way forward for pinning nixpkgs; it autogenerates a list of revisions used to pin, and allows easy updating. Flakes and niv, to a large degree, are about solving the same problem.

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