I am trying to build Future.hs from plutus-use-case from scratch. I take two files from plutus-starter: cabal.project as is, and plutus-starter.cabal that I have to edit according to the imports in Future.hs. I get however these errors when I run cabal build:

src/Plutus/Contracts/Future.hs:574:8: error:
    Illegal symbol `forall' in type
    Perhaps you intended to use RankNTypes or a similar language
    extension to enable explicit-forall syntax: forall <tvs>. <type>

I also had to add several other modules e.g. Plutus.Contracts.Currency and for every .hs file with forall, do I have to add {-# LANGUAGE RankNTypes #-}?

1 Answer 1


Yes, the LANGUAGE pragmas are required, best practice is probably following Plutus code examples and having separate pragmas per-file as needed.

It is also possible to allow some extensions globally via .cabal file by using default-extensions, e.g.:


but this should be done with care and can bite later as some extensions are not compatible.

Also instead of adding files locally, try adding plutus-use-cases to the list of your dependencies, then you can import Plutus.Contracts.Currency.

  • I never thought of adding plutus-use-cases thats helpful!
    – Hud
    Aug 5, 2021 at 15:35

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