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1 Convertible: Data-Conversion Protocol

 (require file/convertible)

The file/convertible library provides a protocol to mediate between providers of data in different possible formats and consumers of the formats. For example, a datatype that implements prop:convertible might be able to convert itself to a GIF or PDF stream, in which case it would produce data for 'gif-bytes or 'pdf-bytes requests.

Any symbol can be used for a conversion request, but the following should be considered standard:

  • 'text — a string for human-readable text

  • 'gif-bytes — a byte string containing a GIF image encoding

  • 'png-bytes — a byte string containing a PNG image encoding

  • 'ps-bytes — a byte string containing a PostScript document

  • 'eps-bytes — a byte string containing an Encapsulated PostScript document

  • 'pdf-bytes — a byte string containing a PDF document

  • 'pdf-bytes+bounds — a list containing a byte string and four numbers; the byte string contains a PDF document and the four numbers are sizing information for the PDF document, namely the width, height, ascent and descent in that order

value

prop:convertible : struct-type-property?

A property whose value should be a procedure of three arguments. The procedure is called when a structure with the property is passed to convert; the first argument to the procedure is the structure, the second argument is a symbol for the requested conversion, and the third argument is a value to return (typically #f if the conversion is not supported. The procedure’s result depends on the requested conversion.

procedure

(convertible? v) → boolean?

  v : any/c
Returns #t if v supports the conversion protocol, #f otherwise.

procedure

(convert v request [default])

 → 
(case request
  [(text) (or/c string? (λ (x) (eq? x default)))]
  [(gif-bytes png-bytes ps-bytes eps-bytes pdf-bytes)
   (or/c bytes? (λ (x) (eq? x default)))]
  [(pdf-bytes+bounds) (or/c (list/c bytes?
                                    (and/c real? (not/c negative?))
                                    (and/c real? (not/c negative?))
                                    (and/c real? (not/c negative?))
                                    (and/c real? (not/c negative?)))
                            (λ (x) (eq? x default)))]
  [else any/c])
  v : convertible?
  request : symbol?
  default : any/c = #f
Requests a data conversion from v, where request indicates the type of requested data and default is the value that the converter should return if it cannot produce data in the format indicated by request.

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