Define eltype only for type

[devmotion]

See https://docs.julialang.org/en/v1/base/collections/#Base.eltype

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.66%. Comparing base (0479a8a) to head (4b35eb2).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
- Coverage   86.84%   86.66%   -0.18%     
==========================================
  Files           1        1              
  Lines          76       75       -1     
==========================================
- Hits           66       65       -1     
  Misses         10       10              

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