[makapuf]

Unless you import rand() your code should be deterministic. You're right about needing to run the thing to get the data (that's the point) but there is a middle ground between pure literals and fully side effects code. By example you could impose pure functions (no side effects).

[lmm]

That's exactly what Dhall is doing.

[marcosdumay]

That's what Haskell already does. Dhall is optimizing on different dimensions (making sure the script execution ends, making the scripts verifiable at static time, making it convenient to marge files, making it convenient to centralize your configuration).