[ADP-2565] Implement SQL expressions for WHERE clauses

[HeinrichApfelmus]

This pull request

• Implements facilities for working with variables: a type VarName, a type Bindings, and a Lets monad.
• Implements a type Expr a that represents expression in the SQL language. Specifically, Expr Bool can be used in a WHERE clause.
• Implements a function selectWhere that works with the Table type and computes a SELECT … WHERE clause.
• Implements a function deleteWhere that works with the Table type

Comments

• I have skipped extensive property testing for now, as I want to stabilize the API first. The module Demo.Database provides a basic functionality check.

Issue Number

ADP-2565

[Anviking]

👍

[Anviking]

Worth making VarName a newtype to distinguish var from text?

[HeinrichApfelmus]

🤔 Yes. Making the type VarName abstract is a good idea, because the only sensible way to create variable names should be newVarName anyway.