Add a few doctest examples for randomR. Disable deps for unrunndabl…

…e doctests

random.cabal

@@ -137,13 +137,15 @@ test-suite doctests
     default-language: Haskell2010
     build-depends:
         base,
-        doctest >=0.15 && <0.19,
-        mwc-random >=0.13 && <0.16,
-        primitive >=0.6 && <0.8,
-        random,
-        stm,
-        unliftio >=0.2 && <0.3,
-        vector >= 0.10 && <0.14
+        doctest >=0.15 && <0.19
+    if impl(ghc >= 8.2) && impl(ghc < 8.10)
+        build-depends:
+            mwc-random >=0.13 && <0.16,
+            primitive >=0.6 && <0.8,
+            random,
+            stm,
+            unliftio >=0.2 && <0.3,
+            vector >= 0.10 && <0.14
 
 test-suite spec
     type:             exitcode-stdio-1.0

src/System/Random.hs

@@ -221,12 +221,23 @@ class Random a where
   -- closed interval /[lo,hi]/, together with a new generator. It is unspecified
   -- what happens if /lo>hi/, but usually the values will simply get swapped.
   --
+  -- >>> let gen = mkStdGen 2021
+  -- >>> fst $ randomR ('a', 'z') gen
+  -- 't'
+  -- >>> fst $ randomR ('z', 'a') gen
+  -- 't'
+  --
   -- For continuous types there is no requirement that the values /lo/ and /hi/ are ever
   -- produced, but they may be, depending on the implementation and the interval.
   --
   -- There is no requirement to follow the @Ord@ instance and the concept of range can be
   -- defined on per type basis. For example product types will treat their values
-  -- independently. In case when a lawful range is desired `uniformR` should be used
+  -- independently:
+  --
+  -- >>> fst $ randomR (('a', 5.0), ('z', 10.0)) $ mkStdGen 2021
+  -- ('t',6.240232662366563)
+  --
+  -- In case when a lawful range is desired `uniformR` should be used
   -- instead.
   --
   -- @since 1.0.0

test/doctests.hs

@@ -10,6 +10,7 @@ main = doctest ["src"]
 
 #else
 
+-- Also disabled in cabal file.
 -- TODO: fix doctest support
 main :: IO ()
 main = putStrLn "\nDoctests are not supported for older ghc version\n"