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Beta-Reduce Trivial Conditionals #1692

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deusaquilus opened this issue Nov 3, 2019 · 1 comment · May be fixed by #1693
Open

Beta-Reduce Trivial Conditionals #1692

deusaquilus opened this issue Nov 3, 2019 · 1 comment · May be fixed by #1693

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@deusaquilus
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deusaquilus commented Nov 3, 2019 •
edited

Version: (e.g. 3.4.8)
Module: (e.g. quill-core)
Database: (e.g. ALL)

As is well known, using a conditional to select a query based on a runtime variable does not work for compile-time queries.

val q = if (cond) quote(query[Person].filter(_.name == "Joe")) else quote(query[Person]) 
q: Quoted[EntityQuery[Person]]{def ast: io.getquill.ast.Query with Product with Serializable} = querySchema("Person").filter(x1 => x1.name == "Joe")

run(q) 
cmd9.sc:1: Dynamic query
val res9 = run(q)

One way to solve this issue however would be to introduce a beta-reduction that would select the then or else clause of a If AST if it sees that the value of condition is trivially true e.g. if it's Constant("foo") == Constant("foo"). That way it would be possible to use conditionals to choose one Query versus another inside of a quotation.

val q = quote { 
  (cond:String) => 
    if (cond=="foo") 
      query[Person].filter(_.name == "Joe") 
    else 
      query[Person] 
} 

// doing q("foo") reduces the AST to: query[Person].filter(_.name == "Joe")
run(q("foo"))
// SELECT p.name, p.age FROM Person p where p.name = 'Joe'

@getquill/maintainers
@deusaquilus deusaquilus linked a pull request Nov 4, 2019 that will close this issue
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@deusaquilus
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Now keep in mind that you still wouldn't be able to use runtime variables in these reductions for instance:

val v = "foo"
run(q(lift(v)))

What you would be able to do however, is to use compile-time values to make a choice as to what kind of query to select:

val v = "foo"
if (v == "foo") run(q("foo")) else run(q("bar"))

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[WIP] Add Beta Reductions for Trivial Conditions zio/zio-quill
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@deusaquilus