Comment by abathologist

Mercury feels like if the Ada people wrote Prolog. it's very verbose. you have to declare signatures in separate files and determinism modes. grounding is strictly enforced. it's statically typed. there's no REPL, remarkably.

in exchange, the compiler catches a lot of bugs and the code is blazing fast.

Curry is a superset of Haskell. it takes Haskell's pattern matching and makes it extremely general (full unification), extends it to non-determinism with choice points. it does have a REPL, like ghci.

Like Haskell, Curry is lazy. Mercury (like Prolog) uses mostly eager, depth-first evaluation (SLDNF resolution.) Clause order doesn't matter in Curry, which uses a strategy of "needed narrowing" - variables are narrowed when they need to be.

Unlike Mercury (and Prolog), and like Haskell and other FP languages, Curry draws a distinction between function inputs and outputs. You can do relational programming via guards and pattern matching, but it doesn't feel as Prolog-y.

Curry is more niche than Mercury, which is at least being used to build Souffle (a static analysis language built on Datalog), which is actually being used in industry somewhat. But it's a shame because Curry has a lot to offer, especially to Haskellers. They're both worth checking out though.

Not a technical difference, but I think Mercury is somewhat more "commerical" in that it's out of development and can be used in real projects, compared to Curry, which is very much in development.