Deferred table header and column mappings

[richardlawrence]

I'd like table header and column mappings to work with #rows_hash, treating the values in the left column as headers and the values in the right column as column values. In order to support this, I changed Table to defer execution of mappings until #hashes or #diff! is called so that the table can be transposed first if necessary. The only change I made to existing specs was to add a call to #hashes for those specs that expected a mapping to raise an error.

Sorry about the wire_protocol.feature change being part of this. I tried to back out a change from another issue, but can't seem to get it removed completely.

[Mange]

Hmm, but with this it's impossible to map headers multiple times. We have to do it like this now:

def records_table(selector)
   records = some_way_to_get_data(selector)
   records.map_headers! { |header| header.downcase }

   # map, dammit
   table(records.hashes)
end

def users_table
   users = records_table('table.users')
   users.map_headers! { |header| header.sub(' (?)', '') }

   # map, dammit
   table(users.hashes)
end

The specific mappings here are fictional, but it's along the same lines. Basically, we have a common table type that has some decorative stuff in the text which we filter away. We then have specific tables that have some other columns that need scrubbing.

The deferred mappings should be stored in an array, perhaps? At least make it easier to do the mapping in case several mappings need to be chained. Any of these would suffice (although the last one is a bit ugly):

table.with_mapped_headers # => #<Table>
table.map_headers! { |header, previous_block| previous_block.call(header).downcase }

[lock]

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