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diff --git a/users/grfn/xanthous/src/Xanthous/Util/Optparse.hs b/users/grfn/xanthous/src/Xanthous/Util/Optparse.hs
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+++ b/users/grfn/xanthous/src/Xanthous/Util/Optparse.hs
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+--------------------------------------------------------------------------------
+module Xanthous.Util.Optparse
+  ( readWithGuard
+  ) where
+--------------------------------------------------------------------------------
+import Xanthous.Prelude
+--------------------------------------------------------------------------------
+import qualified Options.Applicative as Opt
+--------------------------------------------------------------------------------
+
+readWithGuard
+  :: Read b
+  => (b -> Bool)
+  -> (b -> String)
+  -> Opt.ReadM b
+readWithGuard predicate errmsg = do
+  res <- Opt.auto
+  unless (predicate res)
+    $ Opt.readerError
+    $ errmsg res
+  pure res