"implementation is left-biased" - but fromList(With) is not

[jwaldmann]

The actual behaviour of fromListWith surprised me, given the general "left-biased" feeling of the library.
I think I was expecting that these be equivalent

fromListWith (++) [ (0,"foo"), (1,"bar"), (0,"oof") ]
unionsWith (++) [ singleton 0 "foo", singleton 1 "bar", singleton 0 "oof" ]

but they are not. Perhaps add a warning in the documentation of fromListWith? Currently, you have to parse the example to infer the semantics.

[treeowl]

Yes, this trips me up too. The idea is that it does a left fold with
insert, which indeed is unexpected. Feel free to open a PR, or I'll try to
fix it myself.

On Sep 4, 2016 5:32 AM, "jwaldmann" [email protected] wrote:

The actual behaviour of fromList surprised me, given the general
"left-biased" feeling of the library.
I think I was expecting that these be equivalent

fromListWith (++) [ (0,"foo"), (1,"bar"), (0,"oof") ]
unionsWith (++) [ singleton 0 "foo", singleton 1 "bar", singleton 0 "oof" ]

but they are not. Perhaps add a warning in the documentation of fromListWith?
Currently, you have to parse the example to infer the semantics.

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[sjakobi]

Related: haskell-unordered-containers/unordered-containers#264