map transform on array in spark sql

[Koert Kuipers]

i have a DataFrame where one column has type:

ArrayType(StructType(Seq(
  StructField("a", typeA, nullableA),
  StructField("b", typeB, nullableB)
)))

i would like to map over this array to pick the first element in the
struct. so the result should be a ArrayType(typeA, nullableA). i realize i
can do this with a scala udf if i know typeA. but what if i dont know typeA?

basically i would like to do an expression like:
map(col("x"), _(0)))

any suggestions?