diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,9 @@
 # Revision history for dec
 
+## 0.0.4
+
+- Mark module as explicitly `Safe`.
+
 ## 0.0.3
 
 - First version. Released on an unsuspecting world.
diff --git a/dec.cabal b/dec.cabal
--- a/dec.cabal
+++ b/dec.cabal
@@ -1,17 +1,17 @@
 cabal-version:      >=1.10
 name:               dec
-version:            0.0.3
+version:            0.0.4
 synopsis:           Decidable propositions.
 category:           Data, Dependent Types
 description:
   This package provides a @Dec@ type.
   .
   @
-  type Not a = a -> Void
+  type Neg a = a -> Void
   .
   data Dec a
   \    = Yes a
-  \    | No (Not a)
+  \    | No (Neg a)
   @
 
 homepage:           https://github.com/phadej/vec
@@ -20,23 +20,36 @@
 license-file:       LICENSE
 author:             Oleg Grenrus <oleg.grenrus@iki.fi>
 maintainer:         Oleg.Grenrus <oleg.grenrus@iki.fi>
-copyright:          (c) 2019 Oleg Grenrus
+copyright:          (c) 2019-2021 Oleg Grenrus
 build-type:         Simple
 extra-source-files: ChangeLog.md
 tested-with:
-  GHC ==8.8.1 || ==8.6.5 || ==8.4.4 || ==8.2.2 || ==8.0.2 || ==7.10.3 || ==7.8.4
+  GHC ==7.8.4
+   || ==7.10.3
+   || ==8.0.2
+   || ==8.2.2
+   || ==8.4.4
+   || ==8.6.5
+   || ==8.8.4
+   || ==8.10.3
+   || ==9.0.1
 
 source-repository head
   type:     git
   location: https://github.com/phadej/vec.git
+  subdir:   dec
 
 library
+  default-language: Haskell2010
+  hs-source-dirs:   src
+  ghc-options:      -Wall -fprint-explicit-kinds
   exposed-modules:  Data.Type.Dec
-  build-depends:    base >=4.7 && <4.13
+  build-depends:    base >=4.7 && <4.16
 
   if !impl(ghc >=7.10)
-    build-depends: void >=0.7.2 && <0.8
+    build-depends: void >=0.7.3 && <0.8
 
-  ghc-options:      -Wall -fprint-explicit-kinds
-  hs-source-dirs:   src
-  default-language: Haskell2010
+  if impl(ghc >= 9.0)
+    -- these flags may abort compilation with GHC-8.10
+    -- https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3295
+    ghc-options: -Winferred-safe-imports -Wmissing-safe-haskell-mode
diff --git a/src/Data/Type/Dec.hs b/src/Data/Type/Dec.hs
--- a/src/Data/Type/Dec.hs
+++ b/src/Data/Type/Dec.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE Safe #-}
 module Data.Type.Dec (
     -- * Types
     Neg,
@@ -15,7 +16,7 @@
     decToBool,
     ) where
 
-import Data.Void          (Void)
+import Data.Void (Void)
 
 -- | Intuitionistic negation.
 type Neg a = a -> Void