Changes between Version 21 and Version 22 of ViewPatterns
 Timestamp:
 Feb 9, 2007 8:01:11 AM (10 years ago)
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ViewPatterns
v21 v22 1 ;; This buffer is for notes you don't want to save, and for Lisp evaluation. 2 ;; If you want to create a file, visit that file with Cx Cf, 3 ;; then enter the text in that file's own buffer. 4 1 5 [[PageOutline]] 2 6 = View patterns: lightweight views for Haskell = … … 272 276 }}} 273 277 Of course, the argument does not need to be a constant as it is here. 274 275 The (n+k) patterns can be implemented (with different syntax, of course) with a view function that tests for values greater than or equal to n:276 {{{277 np :: Num a => a > a > Maybe a278 np k n  k <= n = Just (nk)279  otherwise = Nothing280 281 f :: Num a => a > Int282 f (np 10 > n) = n  Matches values >= 10, f a = (a  10)283 f (np 4 > n) = 1  Matches values >= 4284 f other = 2285 }}}286 278 287 279 With the value input feature, in a sense, patterns become first class. For example, one could pass a pattern as an argument to a function thus: … … 402 394 The majority of the proposals allow nesting. 403 395 396 397 === Integration with type classes === 398 399 A view mechanism that integrates nicely with type classes would allow 400 a single "view" to decompose multiple different data types. For 401 example, a view might work on any type in class Num, or in class Sequence. 402 403 A good example is Haskell's existing (n+k) patterns. Here is how they 404 can be expressed using the view pattern proposed in this page (with different 405 syntax, of course): 406 {{{ 407 np :: Num a => a > a > Maybe a 408 np k n  k <= n = Just (nk) 409  otherwise = Nothing 410 411 g :: Int > Int 412 g (np 3 > n) = n*2 413 414 h :: Integer > Integer 415 h (np 9 > n) = n*2 416 417 f :: Num a => a > Int 418 f (np 10 > n) = n  Matches values >= 10, f a = (a  10) 419 f (np 4 > n) = 1  Matches values >= 4 420 f other = 2 421 }}} 422 Here a single, overloaded view, `np`, can be used 423 in `g`, and `h` to match against values of different types and, 424 in `f`'s case, any type in class Num. (Notice too the use of the value 425 input feature.) 426 427 This feature falls out very nicely from view patterns, but 428 not from all other proposals. 404 429 405 430 