Unify Expr and SimpleExpr, add ExprTrait?

[Expurple]

Currently, a lot of "operator" methods are duplicated across Expr, SimpleExpr and sea_orm::ColumnTrait, but the list of methods is slightly different for each. The type conversions are also messy and non-obvious. I complained about this in #770 today. Writing generic code on top is especially difficult.

I'd really like to:

• Get rid of Expr / SimpleExpr divide
• Move their methods into a trait (ExprTrait ???) and reuse it for SeaORM columns (ColumnTrait: ExprTrait ???)

I understand that this is a big change that won't happen soon (especially with SeaORM 1.0 on the horizon). But still, let's discuss this possibility

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EDIT: Some other types that would benefit from implementing ExprTrait:

• sea_query::FunctionCall (see Struggling to express `LOWER(expr) IS IN values` (+ many deep issues with expressions) #770)
• sea_query::Value
• sea_orm::Value

[tyt2y3]

I think having an ExprTrait that unifies the API is a pretty good idea. But I don't think it's possible to merge Expr with SimpleExpr. To keep things backwards compatible, we can use the inherent macro.

To begin, can you open a PR and add more doc test examples?

[Expurple]

But I don't think it's possible to merge Expr with SimpleExpr.

Can you share the details? From my point of view as a user, all expressions are usable in all expression contexts and there should be just one type. One of those two is probably an implementation detail. But I may be wrong.

To keep things backwards compatible, we can use the inherent macro.

I like this approach, although it won't work for ColumnTrait methods which accept &self (Expr methods accept self). I think, I'd still use inherent for Expr, then implement ColumnTrait: ExprTrait using something like self.into_expr().trait_method(...) and then switch inherent ColumnTrait methods to use something like self.clone().trait_method(...)

To begin, can you open a PR and add more doc test examples?

Just to clarify: we're good with ExprTrait name? It doesn't need to be changed to something like ExpressionTrait, ExpressionLike, Expression, etc?

[tyt2y3]

I would have preferred the abbreviated version. It's common in Rust. Because Expr is not part of the syntax tree, it's just a builder for SimpleExpr.

[Expurple]

Ok, I'll postpone the Expr/SimpleExpr thing for now and make ExprTrait PRs