CODEWITHSUNDEEP
scalashapeless
visa
visa

Reputation: 320

Shapeless: Why align does not work with singleton types?

I expected this code to compile

import shapeless._
import record._
import ops.record._
import ops.hlist.Align
import syntax.singleton._

case class From(i: Int, s: String, a: Int, b: Int, c: Int)
case class To(j: Int, s1: String, s2: String, a: Int, b: Int, c: Int)
val f = From(1, "FROM", 1, 1, 1)

val fromGen = LabelledGeneric[From]
val toGen = LabelledGeneric[To]

val lgenRepr = fromGen.to(f)
val modified = lgenRepr.renameField('i, 'j) - 's + ('s1 ->> "S1") + ('s2 ->> "S2")

val align = Align[modified.type, toGen.Repr]

toGen.from(align(modified))

but if fails with 

could not find implicit value for parameter alm: shapeless.ops.hlist.Align[modified.type,toGen.Repr]
 val align = Align[modified.type, toGen.Repr]

There is a trick with constructing the needed type using labelled.FieldType, but it doesn't explain how to remove fields, so if it's impossible to make Align work with singleton type, it would be useful to know how to deal with those fields.

Upvotes: 0

Views: 167

Answers (1)

Miles Sabin
Miles Sabin

Reputation: 23046

The problem is that modified.type is a little too precise ... it's the singleton type of that particular val, not the computed record type. To do this on the REPL the following works just fine,

@ modified.align[toGen.Repr] 
res14: toGen.Repr = 1 :: S1 :: S2 :: 1 :: 1 :: 1 :: HNil

Upvotes: 2

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