[Haskell-cafe] Data.Map add only if not exist and return true if
Arthur van Leeuwen
arthurvl at cs.uu.nl
Tue Dec 9 04:13:52 EST 2008
On 9 dec 2008, at 09:11, John Ky wrote:
> I'm looking for a function for Data.Map that will insert a new key
> and value into the map if the key doesn't already exist in the map.
> When the key already exists, I don't want the value updated in the
> map. Additionally, I want to know whether the key/value was
> inserted or not so that I can return that fact as a boolean from my
> I can't work out which function in Data.Map is suitable for this
> task and I wanted to avoid two lookups.
Data.Map.insertLookupWithKey (\k n o -> o)
should do the trick. The first element of the tuple returned
is the Maybe a that is Nothing if no value existed at the given key,
the second element is the updated map.
With kind regards, Arthur.
/\ / | arthurvl at cs.uu.nl | Work like you don't need
/__\ / | A friend is someone with whom | Love like you have never
/ \/__ | you can dare to be yourself | Dance like there's nobody
More information about the Haskell-Cafe