nixpkgs/pkgs/development/interpreters/dhall/build-dhall-package.nix
Gabriel Gonzalez 38f1d70c01 Add Nixpkgs support for Dhall
One of the motivations for this change is the following Discourse
discussion:

https://discourse.dhall-lang.org/t/offline-use-of-prelude/137

Many users have requested Dhall support for "offline" packages
that can be fetched/built/installed using ordinary package
management tools (like Nix) instead of using Dhall's HTTP import system.
I will continue to use the term "offline" to mean Dhall package
builds that do not use Dhall's language support for HTTP imports (and
instead use the package manager's support for HTTP requests, such
as `pkgs.fetchFromGitHub`)

The goal of this change is to document what is the idiomatic way to
implement "offline" Dhall builds by implementing Nixpkgs support
for such builds.  That way when other package management tools ask
me how to package Dhall with their tools I can refer them to how it
is done in Nixpkgs.

This change contains a fully "offline" build for the largest Dhall
package in existence, known as "dhall-packages" (not to be confused
with `dhallPackages`, which is our Nix attribute set containing
Dhall packages).

The trick to implementing offline builds in Dhall is to take
advantage of Dhall's support for semantic integrity checks.  If an
HTTP import is protected by an integrity check and a cached build
product matches the integrity check then the HTTP import is never
resolved and the expression is instead fetched from cache.

By "installing" dependencies in a pre-seeded and isolated cache
we can replace remote HTTP imports with dependencies that have
been built and supplied by Nix instead.

The offline nature of the builds are enforced by compiling the
Haskell interpreter with the `-f-with-http` flag, which disables
the interpreter's support for HTTP imports.  If a user forgets
to supply a necessary dependency as a Nix build product then the
build fails informing them that HTTP imports are disabled.

By default, built packages are "binary distributions", containing
just a cache product and a Dhall expression which can be used to
resolve the corresponding cache product.

Users can also optionally enable a "source distribution" of a package
which already includes the equivalent fully-evaluated Dhall code (for
convenience), but this is disabled by default to keep `/nix/store`
utilization as compact as possible.
2020-02-11 22:02:53 -08:00

84 lines
2.5 KiB
Nix

{ haskell, haskellPackages, lib, lndir, runCommand, writeText }:
{ name
# Expressions to add to the cache before interpreting the code
, dependencies ? []
# A Dhall expression
#
# Carefully note that the following expression must be devoid of uncached HTTP
# imports. This is because the expression will be evaluated using an
# interpreter with HTTP support disabled, so all HTTP imports have to be
# protected by an integrity check that can be satisfied via cached
# dependencies.
#
# You can add a dependency to the cache using the preceding `dependencies`
# option
, code
# `buildDhallPackage` can include both a "source distribution" in
# `source.dhall` and a "binary distribution" in `binary.dhall`:
#
# * `source.dhall` is a dependency-free αβ-normalized Dhall expression
#
# * `binary.dhall` is an expression of the form: `missing sha256:${HASH}`
#
# This expression requires you to install the cache product located at
# `.cache/dhall/1220${HASH}` to successfully resolve
#
# By default, `buildDhallPackage` only includes "binary.dhall" to conserve
# space within the Nix store, but if you set the following `source` option to
# `true` then the package will also include `source.dhall`.
, source ? false
}:
let
# `buildDhallPackage` requires version 1.25.0 or newer of the Haskell
# interpreter for Dhall. Given that the default version is 1.24.0 we choose
# the latest available version instead until the default is upgraded.
#
# HTTP support is disabled in order to force that HTTP dependencies are built
# using Nix instead of using Dhall's support for HTTP imports.
dhall =
haskell.lib.justStaticExecutables
(haskell.lib.appendConfigureFlag
haskellPackages.dhall_1_29_0
"-f-with-http"
);
file = writeText "${name}.dhall" code;
cache = ".cache";
cacheDhall = "${cache}/dhall";
sourceFile = "source.dhall";
in
runCommand name { inherit dependencies; } ''
set -eu
mkdir -p ${cacheDhall}
for dependency in $dependencies; do
${lndir}/bin/lndir -silent $dependency/${cacheDhall} ${cacheDhall}
done
export XDG_CACHE_HOME=$PWD/${cache}
mkdir -p $out/${cacheDhall}
${dhall}/bin/dhall --alpha --file '${file}' > $out/${sourceFile}
SHA_HASH=$(${dhall}/bin/dhall hash <<< $out/${sourceFile})
HASH_FILE="''${SHA_HASH/sha256:/1220}"
${dhall}/bin/dhall encode --file $out/${sourceFile} > $out/${cacheDhall}/$HASH_FILE
echo "missing $SHA_HASH" > $out/binary.dhall
${lib.optionalString (!source) "rm $out/${sourceFile}"}
''