What is the reason for the failure to load the module in Yesod Esqueleto due to it being a part of the hidden package 'esqueleto-3.5.8.1'?

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The reason for the failure to load the module in Yesod Esqueleto due to it being a part of the hidden package 'esqueleto-3.5.8.1' is that the module is not available in the visible package database, and hence it cannot be accessed by Yesod. This is because hidden packages are packages that are not normally used, and are not included in the package database used by GHC (the Glasgow Haskell Compiler) by default. Therefore, to use such packages, you need to explicitly tell GHC where to find them or add them to the visible package database.