(WIP) (RFC) Safer placeholders

[igrep]

Better solution for https://github.com/khibino/haskell-relational-record/pull/67.

Problem

Users must refer placeholders by exactly same order as the record fields are defined, and must refer every field exactly once. Otherwise, the parameter values are bind at unintentional placeholders when executing the SQL.

Solution

1. To Relation and Record, add fields for lists containing indices of every record field.
   • Called placeholderOffsets in Record.
   • The indices for the record fiels are obtained from Pi object generated by makeRelationalRecord.
2. Concatenate the placeholderOffsets of Records by any operators for Records.
3. Accumulate the placeholderOffsets of Records by the new monad transformer: ReferredPlaceholders.
4. Sort parameters by placeholderOffsets before binding to the placeholders of the SQL.

Left problems

• Current implementation can't properly handle the cases that the order of the placeholders referred in the monad stack differs from the order of the placeholders referred in the composed SQL.
  • Maybe I shouldn't have added the ReferredPlaceholders monad transformer.
  • I guess it'd be the best way to add placeholderOffsets to any Columns used in the SubQuery type in some way.
• I'm not sure how to handle the cases that the parameter types are composed by using the placeholder function several times.
  • e.g. The SQL to get columns of a table in DB2.
• And other concerns of mine are noted by igrep TODO sign.

[igrep]

Current implementation can't properly handle the cases that the order of the placeholders referred in the monad stack differs from the order of the placeholders referred in the composed SQL.

Decided by oral discussion with @khibino:

• Wrap every monad transformer accumulating Records (e.g. Restrictings) with ReferredPlaceholders:

  newtype Restrictings c m a =
  Restrictings (ReferredPlaceholders (WriterT (DList (Predicate c)) m a))
  ...
  newtype Assignings r m a =
  Assignings (ReferredPlaceholders (WriterT (Table r -> DList Assignment) m a))

• Wrap every field in the value constructor of SubQuery containing Column (e.g. Tuple, JoinProduct, [Predicate Flat] etc.) with WithPlaceholderOffsets
• Concatenate placeholder offsets composing StringSQL from SubQuery

[igrep]

I'm not sure how to handle the cases that the parameter types are composed by using the placeholder function several times.

Decided by oral discussion with @khibino:

• Add a new function to make users use placeholder only once:
  • i.e. relationWithPlaceholder = relation' . placeholder
• Deprecate calling directly placeholder.
• Make query' and queryList' receive a value representing placeholder offsets with its type.
  • How should I make it compatible with Record?

[igrep]

New task:

• Make every operaton for Record monadic to record PlaceholderOffsets so that Record doesn't have PlaceholderOffsets by itself.

I'm going to rewrite this patch to send a separate pull request again! 😰