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diff --git a/hadrian/src/Expression/Type.hs b/hadrian/src/Expression/Type.hs
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+module Expression.Type where
+
+import Context.Type
+import Way.Type
+
+import Builder
+import qualified Hadrian.Expression as H
+
+-- | @Expr a@ is a computation that produces a value of type @Action a@ and can
+-- read parameters of the current build 'Target'.
+type Expr a = H.Expr Context Builder a
+
+-- | The following expressions are used throughout the build system for
+-- specifying conditions ('Predicate'), lists of arguments ('Args'), 'Ways'
+-- and 'Packages'.
+type Predicate = H.Predicate Context Builder
+type Args      = H.Args      Context Builder
+type Ways      = Expr [Way]