[Proposal] add view function for NPlusKPatterns
Hello, I'm working on a desugaring library for Haskell. Desugaring NPlusKPatterns currently takes up the most code, since a proper translation involves generating two guards. Meanwhile, most other pattern-related language extensions are defined in terms of ViewPatterns, which are trivial to desugar. In my laziness to fix this otherwise, I propose that the following function is added to the Prelude:
maySubtractIntegral :: Integral a => a -> a -> Maybe a maySubtractIntegral k nplusk = if n >= 0 then Just n else Nothing where n = nplusk - k
So that `NPlusKPatterns` like < func (n+42) = expr may be desugared to < func (maySubtractIntegral 42 -> Just n) = expr rather than < func nplusk | n <- nplusk - 42, n >= 0 = expr The name of this function is, of course, subject to bikeshedding. Note that the signature is correct; though (Ord a, Num a) is more general, it's too weak for the semantics of the language extension. Discussion period: 2 weeks
On 2013-09-30 16:37, Stijn van Drongelen wrote:
I'm working on a desugaring library for Haskell. Desugaring NPlusKPatterns currently takes up the most code, since a proper translation involves generating two guards. Meanwhile, most other pattern-related language extensions are defined in terms of ViewPatterns, which are trivial to desugar.
In my laziness to fix this otherwise, I propose that the following function is added to the Prelude: [...]
1. Adding things to the Prelude is not something you can or should do easily. Adding means it has to become part of the Haskell report (i.e. Haskell the language), and removing it takes great effort because many modules may depend on it. 2. If things *are* added to the current Prelude, they should be widely used, universally accepted, and very useful. I don't think your functionality matches any of these. 3. n+k-patterns are not part of the Haskell 2010 standard; if you really need them for compatibility reasons or so, there's a legacy language extension. Using these patterns is universally (?) disencouraged. 4. "My laziness" and golfing are not very good bases for a proposal. David
Am 30.09.2013 16:56, schrieb David Luposchainsky:
On 2013-09-30 16:37, Stijn van Drongelen wrote:
I'm working on a desugaring library for Haskell. Desugaring NPlusKPatterns currently takes up the most code, since a proper translation involves generating two guards. Meanwhile, most other pattern-related language extensions are defined in terms of ViewPatterns, which are trivial to desugar.
In my laziness to fix this otherwise, I propose that the following function is added to the Prelude: [...]
4. "My laziness" and golfing are not very good bases for a proposal.
Stijn, you might add the utility function to a custom library that is bundled with your preprocessor.
On Mon, Sep 30, 2013 at 5:00 PM, Henning Thielemann < schlepptop@henning-thielemann.de> wrote:
Am 30.09.2013 16:56, schrieb David Luposchainsky:
On 2013-09-30 16:37, Stijn van Drongelen wrote:
I'm working on a desugaring library for Haskell. Desugaring NPlusKPatterns currently takes up the most code, since a proper translation involves generating two guards. Meanwhile, most other pattern-related language extensions are defined in terms of ViewPatterns, which are trivial to desugar.
In my laziness to fix this otherwise, I propose that the following function is added to the Prelude: [...]
4. "My laziness" and golfing are not very good bases for a proposal.
Stijn, you might add the utility function to a custom library that is bundled with your preprocessor.
I immediately regret my decision of mailing this proposal to libraries@. You're right :)
participants (3)
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David Luposchainsky -
Henning Thielemann -
Stijn van Drongelen