$ nix-shell | xsel -b
these derivations will be built:
  /nix/store/20hirzhz0i127yzxmajc4lcybm0q5fkb-ihaskell-0.9.1.0.drv
  /nix/store/j73z0ladyq4yrw38m55k5pn7d4m9hcha-statistics-0.15.0.0.drv
  /nix/store/rx3mwb7zpds9k6ir5rzvfj3lcjxngd45-ihaskell-hvega-0.2.0.3.drv
  /nix/store/sibyb0yi8saw8bla0an844qza2d21xi4-monad-bayes-0.1.0.0.drv
  /nix/store/g49j5s00kn2yknv1g8151hs9ibf4ja48-ghc-8.4.4-with-packages.drv
  /nix/store/x0k9rww61qayljywdb4pyncxasmdb4al-ihaskell.drv
  /nix/store/26gr83zalnkrfjhciqdfxm3fnnbzhzr0-ihaskell-kernel.drv
  /nix/store/5x3wj8zijkffvwrrd7f39c88p5k5b8gg-jupyterlab-extended.drv
  /nix/store/9xbbdf7hcgh6sxil2vg6f1js6gfx8gjm-python3.7-jupyterlab-0.35.4.drv
building '/nix/store/5x3wj8zijkffvwrrd7f39c88p5k5b8gg-jupyterlab-extended.drv'...
building '/nix/store/20hirzhz0i127yzxmajc4lcybm0q5fkb-ihaskell-0.9.1.0.drv'...
building '/nix/store/j73z0ladyq4yrw38m55k5pn7d4m9hcha-statistics-0.15.0.0.drv'...
setupCompilerEnvironmentPhase
Build with /nix/store/rd5iasc26rbi07zmn41krzbpigk8ahzy-ghc-8.4.4.
unpacking sources
unpacking source archive /nix/store/9b8j5vbg94kz2cglzpdy73m2ysic5yc3-statistics-0.15.0.0.tar.gz
source root is statistics-0.15.0.0
setting SOURCE_DATE_EPOCH to timestamp 1539291821 of file statistics-0.15.0.0/tests/utils/Makefile
patching sources
compileBuildDriverPhase
setupCompileFlags: -package-db=/build/setup-package.conf.d -j4 -threaded
installing
[1 of 1] Compiling Main             ( Setup.lhs, /build/Main.o )
Linking Setup ...
Node v8.15.0

> /nix/store/35gq5y1l3b5fh0nf95h0izwaiyb7yvwi-nodejs-8.15.0/bin/npm pack jupyterlab-ihaskell
npm ERR! code ENOTFOUND
npm ERR! errno ENOTFOUND
npm ERR! network request to https://registry.npmjs.org/jupyterlab-ihaskell failed, reason: getaddrinfo ENOTFOUND registry.npmjs.org registry.npmjs.org:443
npm ERR! network This is a problem related to network connectivity.
npm ERR! network In most cases you are behind a proxy or have bad network settings.
npm ERR! network
npm ERR! network If you are behind a proxy, please make sure that the
npm ERR! network 'proxy' config is set properly.  See: 'npm help config'

npm ERR! A complete log of this run can be found in:
npm ERR!     /build/.npm/_logs/2019-09-30T13_41_25_368Z-debug.log

Errored, use --debug for full output:
ValueError: "jupyterlab-ihaskell" is not a valid npm package
builder for '/nix/store/5x3wj8zijkffvwrrd7f39c88p5k5b8gg-jupyterlab-extended.drv' failed with exit code 1
cannot build derivation '/nix/store/9xbbdf7hcgh6sxil2vg6f1js6gfx8gjm-python3.7-jupyterlab-0.35.4.drv': 1 dependencies couldn't be built
error: build of '/nix/store/26gr83zalnkrfjhciqdfxm3fnnbzhzr0-ihaskell-kernel.drv', '/nix/store/9xbbdf7hcgh6sxil2vg6f1js6gfx8gjm-python3.7-jupyterlab-0.35.4.drv' failed