Why is DecimalType separate from DataType ?

[Manoj Samel]

Spark 1.2

While building schemaRDD using StructType

xxx = new StructField("credit_amount", DecimalType, true) gives error "type
mismatch; found : org.apache.spark.sql.catalyst.types.DecimalType.type
required: org.apache.spark.sql.catalyst.types.DataType"

From
https://spark.apache.org/docs/1.2.0/api/scala/index.html#org.apache.spark.sql.package,
it seems DecimalType = sql.catalyst.types.DecimalType is separate from
DataType = sql.catalyst.types.DataType

Not sure why that is the case? How does one uses Decimal and other types in
StructField?

Thanks,