improve code quality

[ubogdan]

Describe the PR

• refactor field_parser.go
• refactor operation.go

Relation issue
NONE

Additional context
NONE

[codecov]

Codecov Report

Merging #1049 (f6aeca1) into master (e59f457) will increase coverage by 0.17%.
The diff coverage is 100.00%.

❗ Current head f6aeca1 differs from pull request most recent head 24a31ae. Consider uploading reports for the commit 24a31ae to get more accurate results

@@            Coverage Diff             @@
##           master    #1049      +/-   ##
==========================================
+ Coverage   93.71%   93.89%   +0.17%     
==========================================
  Files           8        8              
  Lines        2100     2096       -4     
==========================================
  Hits         1968     1968              
+ Misses         70       67       -3     
+ Partials       62       61       -1     

Impacted Files	Coverage Δ
field_parser.go	100.00% <100.00%> (ø)
operation.go	95.60% <100.00%> (+0.59%)	⬆️

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