compiler errors - Why does this Scala for expression using tuples fail to compile? -

with scala 2.8.1, compiling this:

val t = (40, 2)  println(for ((i, j) <- list(t)) yield + j)  val e: either[string, (int, int)] = right(t) println(e.right.map {   case (i, j) => + j }) println(for ((i, j) <- e.right) yield + j) 

gives this:

test.scala:9: error: constructor cannot instantiated expected type;  found   : (t1, t2)  required: either[nothing,(int, int)] println(for ((i, j) <- e.right) yield + j) 

according programming in scala, expression should equivalent map/case expression, latter compiles. doing wrong, , how should this?

actually, not quite translation happening. may refer this answer more complete guide, case not explicitly mentioned there.

what happens comprehension pattern matching filters non-matching case. example,

for((i, j) <- list((1, 2), 3)) yield (i, j) 

will return list((1, 2)): list[(any, any)], withfilter called first. now, either doesn't seem have withfilter, use filter, , here's actual translation of comprehension:

e.right.filter { case (i, j) => true; case _ => false }.map { case (i, j) => + j } 

which gives same error. problem e.right returns rightprojection, filter on rightprojection[a, b] returns option[either[nothing, b]].

the reason there no such thing "empty" either (or rightprojection), needs encapsulate result on option.

having said that, surprising when looked @ for-comprehension level. think right thing filter return kind of filtered projection instead.