In January, we presented our work on Explicit Level Imports at the Trends in Functional Programming symposium. We’re pleased to announce that the paper was awarded the the John McCarthy Prize for best paper overall!

The paper introduces the ExplicitLevelImports extension to GHC, which gives programmers fine-grained control over which modules and dependencies are required by Template Haskell. For instance, in the following example, the splice import tells the compiler that Control.Lens.TH is needed only at compile time and not at runtime:

import splice Control.Lens.TH (makeLenses)
import App (S)
data T = MkT { foo :: S }
$(makeLenses ''T )

By taking advantage of this extra information, the compiler can perform less work in certain situations. In one benchmark, we modified pandoc to use ExplicitLevelImports and compilation time was halved when using -fno-code. For full details, read the paper or check out GHC Proposal #682. The implementation has landed in GHC MR !14241, so ExplicitLevelImports will be available in the next major release of GHC (9.14).

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