“Dependencies are, by nature, impure. They're either non-deterministic, have side-effects, or both. Pure functions can't call impure functions (because that would make them impure as well), so pure functions can't have dependencies. Functional programming must reject the notion of dependencies. Obviously, software is only useful with impure behaviour, so instead of injecting dependencies, functional programs must be composed in impure contexts. Impure functions can call pure functions, so at the boundary, an application must gather impure data, and use it to call pure functions. This automatically leads to the ports and adapters architecture.”

“A common pattern for Haskell: First, gather data from impure sources. Second, pass pure data to pure functions. Third, take the pure output from the pure functions, and do something impure with it.”

Great article describing why dependency injection usually makes functions impure due to impure dependencies.