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documentation: add sub-section for the nixpkgs manual Haskell section

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Susan Potter 2018-06-06 13:34:08 -05:00
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doc/languages-frameworks/haskell.section.md
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@ -716,6 +716,62 @@ sets at once:
}
```
### How to specify source overrides for your Haskell package
When starting a Haskell project you can use `developPackage`
to define a derivation for your package at the `root` path
as well as source override versions for Hackage packages, like so:
```nix
# default.nix
{ compilerVersion ? "ghc842" }:
let
# pinning nixpkgs using new Nix 2.0 builtin `fetchGit`
pkgs = import (fetchGit (import ./version.nix)) { };
compiler = pkgs.haskell.packages."${compilerVersion}";
pkg = compiler.developPackage {
root = ./.;
source-overrides = {
# Let's say the GHC 8.4.2 haskellPackages uses 1.6.0.0 and your test suite is incompatible with >= 1.6.0.0
HUnit = "1.5.0.0";
};
};
in pkg
```
This could be used in place of a simplified `stack.yaml` defining a Nix
derivation for your Haskell package.
As you can see this allows you to specify only the source version found on
Hackage and nixpkgs will take care of the rest.
You can also specify `buildInputs` for your Haskell derivation for packages
that directly depend on external libraries like so:
```nix
# default.nix
{ compilerVersion ? "ghc842" }:
let
# pinning nixpkgs using new Nix 2.0 builtin `fetchGit`
pkgs = import (fetchGit (import ./version.nix)) { };
compiler = pkgs.haskell.packages."${compilerVersion}";
pkg = compiler.developPackage {
root = ./.;
source-overrides = {
HUnit = "1.5.0.0"; # Let's say the GHC 8.4.2 haskellPackages uses 1.6.0.0 and your test suite is incompatible with >= 1.6.0.0
};
};
# in case your package source depends on any libraries directly, not just transitively.
buildInputs = [ zlib ];
in pkg.overrideAttrs(attrs: {
buildInputs = attrs.buildInputs ++ buildInputs;
})
```
Notice that you will need to override (via `overrideAttrs` or similar) the
derivation returned by the `developPackage` Nix lambda as there is no `buildInputs`
named argument you can pass directly into the `developPackage` lambda.
### How to recover from GHC's infamous non-deterministic library ID bug
GHC and distributed build farms don't get along well: