Tweak docs

[ghc-base.git] / Control / Concurrent / QSem.hs

diff --git a/Control/Concurrent/QSem.hs b/Control/Concurrent/QSem.hs
index 87f5543..cc78470 100644 (file)
--- a/Control/Concurrent/QSem.hs
+++ b/Control/Concurrent/QSem.hs
@@ -39,11 +39,14 @@ newtype QSem = QSem (MVar (Int, [MVar ()]))
 
 INSTANCE_TYPEABLE0(QSem,qSemTc,"QSem")
 
--- |Build a new 'QSem'
+-- |Build a new 'QSem' with a supplied initial quantity.
+--  The initial quantity must be at least 0.
 newQSem :: Int -> IO QSem
-newQSem initial = do
-   sem <- newMVar (initial, [])
-   return (QSem sem)
+newQSem initial =
+    if initial < 0
+    then fail "newQSem: Initial quantity must be non-negative"
+    else do sem <- newMVar (initial, [])
+            return (QSem sem)
 
 -- |Wait for a unit to become available
 waitQSem :: QSem -> IO ()