title: R data.table fread: specify column data type
body: <p>Is it possible to specify the data type of numeric columns in fread? Commands like </p>
  
  <p><code>prices = markets[, fread(paste(mkt, 'price.csv')), by = mkt]</code></p>
  
  <p>will fail if price data are integers in one file and floats in another.</p>
  
  <p>So instead of a clean one-liner, I have to explicitly write a for-loop and cast the data type to numeric and concatenate the results.</p>