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The suggested solution for the addTool function causes the existing function printTools to fail because the value for the lastReturned field is set to NULL after testing addTool.

*** Exception: ConversionFailed {errSQLType = "NULL", errHaskellType = "Day", errMessage = "expecting SQLText column type"}

I worked around this by changing the type of lastReturned from Day to Maybe Day in the Tool type + associated fixes.

Might be helpful to add some treatment in this chapter on how to handle NULLs.