Add Map.merge to FSharp.Core#19994
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Combines two maps into one, using a caller-supplied function to resolve the values of keys present in both maps. The right-biased "last one wins" merge from the original proposal is just a special case of the combiner form (Map.merge (fun _ _ v2 -> v2)). Implements fsharp/fslang-suggestions#560.
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| let merge resolve (table1: Map<_, _>) (table2: Map<_, _>) = | ||
| (table1, table2) | ||
| ||> fold (fun acc key value2 -> | ||
| acc | ||
| |> change key (function | ||
| | Some value1 -> Some(resolve key value1 value2) | ||
| | None -> Some value2)) |
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Regardless of the signature decided on, I think we'd want the implementation to use the internal MapTree<_, _> directly to avoid creating intermediate Map<_, _> values for every entry.
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Now that #19265 (print/printn) is in good shape, I went looking for other small, self-contained additions to FSharp.Core I could help with, and landed on suggestion #560: a
Map.mergefunction. It's approved-in-principle and gets asked for fairly regularly, so it felt worth a shot.Map.mergecombines two maps into one. When a key is present in both, a caller-supplied function resolves the two values:I went with the combiner form rather than the plain "last one wins" from the original 2017 proposal, since right-biased merge is just a trivial special case of it (
Map.merge (fun _ _ v2 -> v2)).One caveat: unlike the print/printn ticket, this suggestion never got a detailed RFC, so the exact shape (name, argument order, combiner vs. right-biased) is really my best guess and probably wants a maintainer's eye before it goes any further.
Suggestion: fsharp/fslang-suggestions#560