I'm sorry if I wasn't clear. With the appropriate types in Haskell, you can write a data type that would be called a free applicative functor. It can be used to

I'm sorry if I wasn't clear. With the appropriate types in Haskell, you can write a data type that would be called a free applicative functor. It can be used to describe the shape of a computation. By reducing the surface area of said functor, by not exposing the "pure" operation, one can describe computation that are fixed in terms of their input. This is amazing, because I mean truly fixed. The course of execution cannot be altered by the values of the input. Cache line eviction? All memory access is statically known in advance. Branch prediction? There are no branches. The code would look a lot like what the Haxl[1] project at Facebook has produced. They used applicative functors to create a domain-specific language that described the shape of a computation without specifying the details. That allows them to write a compiler for their DSL in Haskell. In their case, they used Haxl to hide concurrency from their users, Facebook developers. Those developers can use Haxl to write concurrent, cached, optimized and type-checked queries against Facebook's datastores, without having to know how to write safe concurrent, fast queries. To the Facebook developers, there is no concurrency, the code they write just looks like a monad or an applicative in Haskell. It should be no surprise then that one of the lead developers of Haxl originally worked on the Glasgow Haskell Compiler. [1] Haxl slides [PDF warning]: https://github.com/meiersi/HaskellerZ/raw/master/meetups/20130829-FPAfternoon_The_Haxl_Project_at_Facebook/The%20Haxl%20Project%20at%20Facebook.pdf https://github.com/meiersi/HaskellerZ/raw/master/meetups/201...