Syntax for map field in Canonical Transform


I am trying to write the canonical transform for an entity type with a map field, and I am wondering what the syntax is. I have tried something like the code below and provisioning works fine with no errors, but after uploading and processing the data (with SUCCESS in DataLoadProcessLog), nothing shows up in the mapField. Is there any documentation on the syntax for map fields in canonical transforms?

entity type SomeType schema name "SMTYP" {
    mapField: map<string, double>  schema name MPFLD

type TransformCanonicalSomeTypeToSomeType mixes SomeType transforms CanonicalSomeType {
    mapField:  ~ expression {"canonicalColumnName1": canonicalField1, "canonicalColumnName2": canonicalField2}


Your should work with type json if you can switch from map to json in your target type.