ccellado

Purity and pure FP

• purity is a syntactical property of programs, more precisely defined as referential transparency: replacing an expression for its bound value doesn't change meaning (precise definition)
• side effects are things that break referential transparency (precise definition)
• There's no direct connection between the concepts of IO, State and so on, and side effects. Traditionally, these things are side effectful (break referential transparency), but you can have abstractions that are pure (don't break ref.trans.)
• Due to the above, the term purely functional programming is not an oxymoron, it makes perfect sense.
• In haskell style pure FP, type constructors and algebras are used to represent "things that would otherwise traditionally be side effects (break ref.trans.)". We call these F[_]s effects or computational contexts for brevity. You can see for yourself how this cannot be a precise definition (especially because they could also have kind higher than * -> *, even though most famous a

ccellado

Building a Nix Package (The C&C++ Version)

Nix can be used to build any kind of package. But here I'm just going to focus on the simple C&C++ case.

Firstly we have to know that the final built packages will located inside /nix/store. Which is globally readable directory of all build inputs and build outputs of the Nix system. The emphasis is on readable, not writable, that is /nix/store is meant to be modified by the user or programs except for the Nix system utilities. This centralises the management of packages, and keeps our packages and package configuration consistent.

So what exactly are we trying to build. Our goal is to build a directory that will be located in /nix/store/*-package-version/, where * is the hash of the package. Preferably a version is also available, but some C&C++ packages don't have versions, so in that case, there's only /nix/store/*-package/.

What will be inside this directory? It follows the GNU Coding Standards descri