Add let expression in do notation.

[BowenFu]

No description provided.

[codecov-commenter]

Codecov Report

Merging #30 (47c2c3a) into main (96254e5) will increase coverage by 0.00%.
The diff coverage is 81.81%.

@@           Coverage Diff           @@
##             main      #30   +/-   ##
=======================================
  Coverage   73.43%   73.43%           
=======================================
  Files           1        1           
  Lines         877      896   +19     
  Branches      241      247    +6     
=======================================
+ Hits          644      658   +14     
- Misses         17       18    +1     
- Partials      216      220    +4     

Impacted Files	Coverage Δ
include/hspp.h	73.43% <81.81%> (+<0.01%)	⬆️

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