diff --git a/nix-eval.cabal b/nix-eval.cabal
--- a/nix-eval.cabal
+++ b/nix-eval.cabal
@@ -2,7 +2,7 @@
 -- see http://haskell.org/cabal/users-guide/
 
 name:                nix-eval
-version:             0.1.0.0
+version:             0.1.0.1
 synopsis:            Evaluate Haskell expressions using Nix to get packages
 description:         Evaluate Haskell expressions using Nix to get packages
 homepage:            http://chriswarbo.net/git/nix-eval
diff --git a/src/Language/Eval/Internal.hs b/src/Language/Eval/Internal.hs
--- a/src/Language/Eval/Internal.hs
+++ b/src/Language/Eval/Internal.hs
@@ -73,14 +73,22 @@
                in concat ["haskellPackages.ghcWithPackages ",
                           "(h: [", unwords pkgs, "])"]
 
+-- | Turn an expression into a Haskell module, complete with imports and `main`
 mkHs :: Expr -> String
 mkHs (Expr (_, ms, e)) = unlines (imports ++ [main])
   where imports = map (\(Mod m) -> "import " ++ m) ms
         main    = "main = putStr (" ++ e ++ ")"
 
+-- | Strip leading and trailing whitespace
 trim :: String -> String
 trim = reverse . dropWhile isSpace . reverse . dropWhile isSpace
 
+-- | Check if the `nix-shell` command is available via the shell
+haveNix :: IO Bool
+haveNix = do
+  (c, _, _) <- readCreateProcessWithExitCode (shell "hash nix-shell") ""
+  return (c == ExitSuccess)
+
 -- User-facing combinators
 
 -- | A raw String of Haskell code, with no packages or modules. You can use
@@ -99,11 +107,15 @@
 asString :: (Show a) => a -> Expr
 asString = raw . show
 
+-- | Qualify an expression, eg. `qualified "Data.Bool" "not"` gives the
+--   expression `Data.Bool.not` with "Data.Bool" in its module list
 qualified :: Mod -> String -> Expr
 qualified (Mod m) e = Expr ([], [Mod m], m ++ "." ++ e)
 
+-- | Append modules to an expression's context
 withMods :: [Mod] -> Expr -> Expr
 withMods ms (Expr (ps, ms', e)) = Expr (ps, ms' ++ ms, e)
 
+-- | Append packages to an expression's context
 withPkgs :: [Pkg] -> Expr -> Expr
 withPkgs ps (Expr (ps', ms, e)) = Expr (ps' ++ ps, ms, e)
diff --git a/tests/Main.hs b/tests/Main.hs
--- a/tests/Main.hs
+++ b/tests/Main.hs
@@ -21,17 +21,24 @@
 
 module Main where
 
+import Data.Maybe
 import Language.Eval
+import Language.Eval.Internal (haveNix)
 import Test.QuickCheck.Monadic
 import Test.Tasty (defaultMain, testGroup)
 import Test.Tasty.QuickCheck
 
-main = defaultMain $ testGroup "All tests" [
-    testProperty "Can eval unit"       unitEval
-  , testProperty "Can eval sums"       sumEval
-  , testProperty "Can import modules"  modulesImport
-  , testProperty "Can import packages" packagesImport
-  ]
+-- | Only test if `nix-shell` is available. In particular, when Nix builds and
+--   tests this package, it uses a pure environment where nix-shell isn't
+--   available
+main = do nix <- haveNix
+          defaultMain $ testGroup "All tests" $ if nix then [
+                testProperty "Can eval unit"       unitEval
+              , testProperty "Can eval sums"       sumEval
+              , testProperty "Can import modules"  modulesImport
+              , testProperty "Can import packages" packagesImport
+              ]
+            else []
 
 unitEval = checkIO "()" (Just "()")