Adds insertIfAbsent to SLMap

The `Data.LVar.SLMap.insert` function throws the runtime error if
multiple insertion attempts at the same key index is attempted. This
commits adds `insertIfAbsent`, which silently ignores subsequent
insertion attempts after a first successful attempt for a given key.

Relates to iu-parfunc#124 .

haskell/lvish/Data/LVar/SLMap.hs

@@ -31,7 +31,7 @@ module Data.LVar.SLMap
  -- * The type and its basic operations
  IMap,
  newEmptyMap, newMap, newFromList,
- insert,
+ insert,insertIfAbsent,
  getKey, waitSize, waitValue,
  modify,
 
@@ -238,6 +238,18 @@ insert !key !elm (IMap (WrapLVar lv)) = WrapPar$ putLV lv putter
  Added _ -> return $ Just (key, elm)
  Found _ -> throw$ ConflictingPutExn$ "Multiple puts to one entry in an IMap!"
 
+-- | Put a single entry into the map if the key is not present,
+-- otherwise the insertion request is silently ignored.
+-- (WHNF) Strict in the key and value.
+insertIfAbsent :: (Ord k, Eq v, HasPut e) =>
+ k -> v -> IMap k s v -> Par e s ()
+insertIfAbsent !key !elm (IMap (WrapLVar lv)) = WrapPar$ putLV lv putter
+ where putter slm = do
+ putRes <- SLM.putIfAbsent slm key $ return elm
+ case putRes of
+ Added _ -> return $ Just (key, elm)
+ Found _ -> return Nothing
+
 -- | `IMap`s containing other LVars have some additional capabilities compared to
 -- those containing regular Haskell data. In particular, it is possible to modify
 -- existing entries (monotonically). Further, this `modify` function implicitly