如何在esqueleto查询中进行比较之前应用函数

Question

For a query simple as that

runDb . select . from $ \cell -> do
  where_ $ cell ^. CellCode ==. val "x"
  return cell

I want to apply a function before the comparison of the field value with "x". The reason is that the cell code has trailing spaces in the database and nothing easier than trimming them away, eg with strip from Data.Text . However, my initial approach of using fmap (twice) resulted in

No Instance for (Functor SqlExpr)

I know that there are functions provides by Esqueleto, like just , that accomplish similar things specifically (I couldn't find the implementation of just , though).

Is there a way to apply any function on the packed value?

While writing: in my specific case, I just might want to use like .

EDIT: Added the specific function I want to apply.

Answer 1

What kind of function to you want to apply?

Here is how someone added the ability to call the chr() function in a query:

https://github.com/krisajenkins/esqueleto/commit/fa1d1c888770e297fef52d76b6cb68342a6c0376

If it is a built-in function (or a user-definable function), perhaps you can do something similar.

Answer 2

See here for a post that adds the postgresql function trim :

import Database.Esqueleto.Internal.Sql

trim :: (IsString s) => SqlExpr (Value s) -> SqlExpr (Value s) -> SqlExpr (Value s)
trim pattern target =
    unsafeSqlFunction "trim" (unsafeSqlBinOp "FROM" pattern target)

(If you're not using postgres, you may need to consult the documentation from your database to find if it supports something similar.)

unsafeSqlFunction can be used to import any function your database supports, but it is unsafe because you have the responsibility to make sure the type signature is actually what your database expects. The name will be copied literally to your SQL.

unsafeSqlBinOp is similar, but it defines a binary operation: unsafeSqlBinOp "FROM" "a" "b" is translated into the SQL "a" FROM "b" .

With this, you should be able to do:

runDb . select . from $ \cell -> do
    where_ $ trim " " (cell ^. CellCode) ==. val "x"
    return cell