Now compiles (although some additional debug still required), see https://try.ps.ai/?gist=540b36954ee840f8c32a95bfa4324d1f

db.purs

data Column v = Column String

data Table columnRow = Table {name :: String, columns :: Record columnRow}

data Select tableColumns selectedColumns = Select
  (Record tableColumns -> Record selectedColumns)
  (Table tableColumns)

data ToStr = ToStr

instance toStrColum :: Folding ToStr String (Column t) String where
  folding ToStr acc (Column n) = acc <> ", " <> n

-- instance toStrConst :: Folding ToStr String t String where
--   folding ToStr acc _ = acc <> ", " <> "const"

-- A more general signature is also allowed
toStr :: forall t o. HFoldl ToStr String t o => t -> o
--toStr :: forall t . HFoldl ToStr String t String => t -> String
toStr = hfoldl ToStr ""

toSQL :: forall tableColumns selectedColumns .
  HFoldl ToStr String (Record selectedColumns) String =>
  Select tableColumns selectedColumns -> String
toSQL (Select selector (Table {name, columns})) = 
 let selectedColumnRecord = selector columns :: Record selectedColumns
 in "SELECT " <> toStr selectedColumnRecord <> " FROM " <> name

-- Testing

type TestTableColums = (
  id :: Column Int,
  name :: Column String
  )

testTableColumns :: Record TestTableColums
testTableColumns = {
    id : Column "id" :: Column Int,
    name : Column "name" :: Column String
  }
  
testTable :: Table TestTableColums
testTable = Table {
  name : "TEST",
  columns : testTableColumns
}

selectId :: Record TestTableColums -> Record (id :: Column Int)
selectId = \t -> {id : t.id}

query :: Select TestTableColums (id :: Column Int)
query = Select selectId testTable

test = toSQL query