/usr/share/doc/frown-doc/examples/manual/Reader.lhs is in frown-doc 0.6.1-14.
This file is owned by root:root, with mode 0o644.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 | > module Reader where
>
> newtype Reader env a = Reader { apply :: env -> a }
>
> instance Monad (Reader env) where
> return a = Reader (\ env -> a)
> m >>= k = Reader (\ env -> apply (k (apply m env)) env)
> fail s = Reader (error s)
>
> getenv :: Reader env env
> getenv = Reader (\ env -> env)
>
> withenv :: env -> Reader env a -> Reader env' a
> withenv env m = Reader (\ env' -> apply m env)
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