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diff --git a/src/CabalHelper/Compiletime/Types/RelativePath.hs b/src/CabalHelper/Compiletime/Types/RelativePath.hs
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+++ b/src/CabalHelper/Compiletime/Types/RelativePath.hs
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+-- cabal-helper: Simple interface to Cabal's configuration state
+-- Copyright (C) 2018  Daniel Gröber <cabal-helper@dxld.at>
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU General Public License as published by
+-- the Free Software Foundation, either version 3 of the License, or
+-- (at your option) any later version.
+--
+-- This program is distributed in the hope that it will be useful,
+-- but WITHOUT ANY WARRANTY; without even the implied warranty of
+-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+-- GNU General Public License for more details.
+--
+-- You should have received a copy of the GNU General Public License
+-- along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+{-|
+Module      : CabalHelper.Compiletime.Types.RelativePath
+License     : GPL-3
+-}
+
+module CabalHelper.Compiletime.Types.RelativePath
+    ( RelativePath
+    , mkRelativePath
+    , unRelativePath
+    ) where
+
+import System.FilePath
+
+-- | A path guaranteed to be relative. The constructor is not exposed, use the
+-- 'mkRelativePath' smart constructor.
+newtype RelativePath = RelativePath { unRelativePath :: FilePath }
+
+-- | Smart constructor for 'RelativePath'. Checks if the given path is absolute
+-- and throws 'UserError' if not.
+mkRelativePath :: FilePath -> RelativePath
+mkRelativePath dir
+    | isAbsolute dir = RelativePath dir
+    | otherwise = error "mkRelativePath: the path given was absolute!"