simpleComment | 注释生成器，表格，列表，图片转文本，并搜罗了诸如羊驼，柴犬狗头，龙猫等80多个图形文本

I want to implement the same method as parsec's try method. But instead of using a parse transformer, I am using a Parser object that holds state:

I've created two tables, simplecomments and commentors, and connected them with INNER JOIN.

• Simplecomments is details of each and every commenter, involving their comment, reg_date, commentorid etc...
• Commentors is the personal info of a commenter with following columns: id, name, email..

I've joined them successfully, however I'm finding it hard to delete from the joined table.

I want to make it like this logic:

1. If there's last row of a commentor called --let's say A-- then delete both his/her comment details and A himself/herself from the table.

2. Else if A has commented plenty of times, with different comments, delete his/her comment details, but let his/her personal info remain since A has other comments there.

This is how I've made it:

I've created two tables, simplecomments and commentors, and joined them with INNER JOIN on commentors.id = simplecomments.commentorid.

Commentors is the personal info of a commenter with following columns: number(id), name, email..

Simplecomments is details of each and every commenter, containing comment, reg_date, commentorid, deliverytype, rating... I've joined them successfully, however I'm finding it hard to delete from the joined table.

I want to make it like this:

if there's last row of a commentor called.. let say A, then delete both his comment details and A himself from the table.

Else if A has commented plenty of times, (with different comments), delete his comment details, but let his personal info remain since he, A, has other comments there..

How should I proceed?

EDIT: For those with similar ailments, I found this is related to the "Extensible Records Problem", something I will personally research more into.

EDIT2: I have started to solve this (weeks later now) by being pretty explicit about data types, and having multiple data types per semantic unit of data. For example, if the database holds an X, my code has an XAction for representing things I want to do with an X, and XResponse for relaying Xs to an http client. And then I need to build the supporting code for shuttling bits between instances. Not ideal, but, I like that it's explicit, and hopefully when my models crystallize, it shouldn't really need much up keep, and should be very reliable.

I'm not sure what the correct level of abstraction is for tackling this problem (ie records? or Yesod?) So I'll just lay out the simple case.

I want to decode a request body into a type

data Comment = Comment {userid :: ..., comment :: ...}

but actually I don't want the request body to contain userid, the server will supply that based on their Auth Headers, (or wherever I want to get data to default fill a field).

So they actually pass me something like:

data SimpleComment = SimpleComment {comment :: ...} deriving (Generic, FromJSON)

And I turn it into a Comment. But maintaining both nearly-identical types simultaneously is a hassle, and not DRY.

How do I solve this problem?

I have a record type:

data Comment = Comment {userid :: ..., comment :: ...}

I have a POST route: