[Ur] returnBlob behavior?

Sun Feb 19 10:15:53 EST 2017

I think you've gotten the wrong idea about [returnBlob].  It operates in 
the style of exceptions, ending the transaction with the given result.  
It's not a normal function return, so you can't use it pass a blob from 
one part of your program to another.  Also, it definitely doesn't 
generate strings.

Somewhat intentionally, Ur/Web currently does not provide a way to 
introspect into a blob, without using the FFI.  It would take some 
thought on what the right API would be.  In my applications that involve 
CSV import, I have their content pasted into textboxes.

On 02/19/2017 10:11 AM, Marko Schütz Schmuck wrote:
> I'm confused about the use of returnBlob.
>
> I was looking through the existing urweb demos to see how I could
> access the data in a blob of mime type "text/plain" as a string.
> Eventually, I want to import into a table from a CSV file the user
> uploads.
>
> I hoped that returnBlob used in a transaction string could achieve
> this. So I modified demo/upload.ur to
>
> fun echo (r : {File : file}) : transaction string =
>    if blobSize (fileData r.File) > 100000 then
>        return "Whoa!  That one's too big."
>    else
>        returnBlob (fileData r.File) (blessMime (fileMimeType r.File))
>
> fun echoLength (r : {File : file}) : transaction page =
>      s <- echo r ;
>      return <xml>{[String.length s]}</xml>
>
> fun main () : transaction page
>    = return <xml><body>
>    <h1>The Amazing File Echoer!</h1>
>
>    <form>Upload a file: <upload{#File}/> <submit action={echoLength}/></form>
> </body></xml>
>
> This compiles and runs, and when I supply a file longer than 100000 I
> get the expected page with the result 26, but to my surprise a shorter
> file results in a page with the content of the file not it's length.
>
> I'd really appreciate some help.
>
> Thanks in advance and best regards,
>
> Marko