feat: add AssignBy

[yoshidan]

I need the function to customize map merging.

[codecov-commenter]

Codecov Report

Merging #164 (3e6bf66) into master (05d69d7) will increase coverage by 0.08%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #164      +/-   ##
==========================================
+ Coverage   89.89%   89.97%   +0.08%     
==========================================
  Files          12       12              
  Lines        1326     1337      +11     
==========================================
+ Hits         1192     1203      +11     
  Misses        132      132              
  Partials        2        2              

Flag	Coverage Δ
unittests	89.97% <100.00%> (+0.08%)	:arrow_up:

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Impacted Files	Coverage Δ
map.go	100.00% <100.00%> (ø)

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[samber]

Hi @yoshidan and thanks for this first contribution.

I wonder if it would be better to use a variadic callback such as iteratee(args ...T) T, instead of a simple iteratee(a T, b T) T.

I suppose in most cases, it is easier to build a callback with a fixed number of arguments, but sometimes, having all intermediary values would be very useful.

WDYT?

[yoshidan]

Hi @samber and thanks for your review.

I implemented it with reference to mergeWith. Since customzer needs oldValue and srcValue, I thought it would be better to limit the argument of iteratee to two. If it should be general purpose, it would be better to include the key and map itself in the callback like the internal implementation of mergeWith.

iteratee func(V, V, K, map[K]V, map[K]V)

result1 := AssignBy(func(a int, b int, _ string, _ map[string]int, _ map[string]int) int {
     return a + b
}, map[string]int{"a": 1, "b": 2}, map[string]int{"b": 3, "c": 4})

I couldn't think of a use case that I wanted to have variadic arguments, so could you please tell me when it's needed?