Proposal: Remove Semigroup and Monoid instances for Data.Map, Data.IntMap, Data.HashMap

Andrew Martin andrew.thaddeus at
Tue Feb 13 19:43:10 UTC 2018

I am strongly in favor of this proposal. I've actually proposal the same
thing (with additional motivations) on the GHC Trac:

I also agree the Monoid instance should be strict in the values. I'm not
sure that the Monoidless Map phase needs to last four or five years (I was
thinking more like two or three years), but that discussion could happen

On Tue, Feb 13, 2018 at 2:33 PM, David Feuer <david.feuer at> wrote:

> Many people have recognized for years that the Semigroup and Monoid
> instances for Data.Map, Data.IntMap, and Data.HashMap are not so
> great. In particular, when the same key is present in both maps, they
> simply use the value from the first argument, ignoring the other one.
> This somewhat counter-intuitive behavior can lead to bugs. See, for
> example, the discussion in Tim Humphries's blog post[*]. I would like
> do do the following:
> 1. Deprecate the Semigroup and Monoid instances for Data.Map.Map,
> Data.IntMap.IntMap, and Data.HashMap.HashMap in the next releases of
> containers and unordered-containers.
> 2. Remove the deprecated instances.
> 3. After another several years (four or five, perhaps?), make a major
> release of each package in which the instances are replaced with the
> following:
>   instance (Ord k, Semigroup v) => Semigroup (Map k v) where
>     (<>) = Data.Map.Strict.unionWith (<>)
>   instance (Ord k, Semigroup v) => Monoid (Map k v) where
>     mempty = Data.Map.Strict.empty
>   instance Semigroup v => Semigroup (IntMap v) where
>     (<>) = Data.IntMap.Strict.unionWith (<>)
>   instance Semigroup v => Monoid (IntMap v) where
>     mempty = Data.IntMap.Strict.empty
>   instance (Eq k, Hashable k, Semigroup v) => Semigroup (HashMap k v) where
>     (<>) = Data.HashMap.Strict.unionWith (<>)
>   instance (Eq k, Hashable k, Semigroup v) => Monoid(HashMap k v) where
>     mempty = Data.HashMap.Strict.empty
> Why do I want the strict versions? That choice may seem a bit
> surprising, since the data structures are lazy. But the lazy versions
> really like to leak memory, making them unsuitable for most practical
> purposes.
> The big risk:
> Someone using old code or old tutorial documentation could get subtly
> wrong behavior without noticing. That is why I have specified an
> extended period between removing the current instances and adding the
> desired ones.
> Alternatives:
> 1. Remove the instances but don't add the new ones. I fear this may
> lead others to write their own orphan instances, which may not even
> all do the same thing.
> 2. Write separate modules with newtype-wrapped versions of the data
> structures implementing the desired instances. Unfortunately, this
> would be annoying to maintain, and also annoying to use--packages
> using the unwrapped and wrapped versions will use different types.
> Manually wrapping and unwrapping to make the different types work with
> each other will introduce lots of potential for mistakes and
> confusion.
> Discussion period: three weeks.
> [*]
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-Andrew Thaddeus Martin
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