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karamaan / karamaan-opaleye

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ShowConstant: showThrough and TH for generating instances for newtypes #51

Closed bergmark closed 10 years ago

bergmark commented 10 years ago

TH is in a separate module, as it is for RunQuery. Id prefer if the top level module for each would re-export the TH functions but that requires moving things around a bit so I didn't do that.

Closes #50

tomjaguarpaw commented 10 years ago

This looks good. The fact that the typeclass constraint is restricted to showConstant a a (i.e. the type variables are the same on both sides) suggests that there's something better to be found, but this functionality and the TH will help to reduce boiler plate in the interim.

bergmark commented 10 years ago

What's a use case where you want ShowConstant a b? If you want to be able to compare a text column where the haskell representation is Text one one side and String on the other?

tomjaguarpaw commented 10 years ago

ShowConstant a b wouldn't help with comparing Wire Text with Wire String.

The use case is when you have something like a Wire ProductId field, where the underlying Postgres datatype is a text. You might then want ShowConstant String ProductId and ShowConstant Text ProductId. However, it's also arguable that you should only have ShowConstant ProductId ProductId, so we still need to play around with the design a bit and see what works best I think.