[Haskell-cafe] What does "1 = 2" mean in Haskell?

[Harendra Kumar]

In these examples, we can identify the constructor (capitalized first
letter) on the LHS and so we are trained to know that it is a pattern
match. The original point related to number specialness was that "1 = 2" is
not easily identifiable as a pattern match because there are no explicit
constructors. The literal "1" here is neither an "explicit constructor" nor
a binding symbol.

-harendra

On 24 February 2017 at 10:48, Jeff Clites <jclites at mac.com> wrote:

> This works too:
>
>     Nothing = Just "hello"
>
> so you get the same effect even without any literal number specialness.
>
> Even this:
>
>     Just x = Nothing
>
> also "works" until you force evaluation of x, as an irrefutable (lazy)
> pattern match. So in a way, you could view the first case as a lazy pattern
> match in which there is nothing you could possibly force, so there's no way
> to manifest the pattern match failure.
>
> Just another way of looking at it.
>
> JEff
>
> On Feb 23, 2017, at 7:00 PM, Brandon Allbery <allbery.b at gmail.com> wrote:
>
> It is, yes. (Literal numbers in patterns occasionally have unexpected type
> ramifications as a result; and occasionally others, since the compiler
> rewrites the pattern match into a guard. It's one of those things that Just
> Works 99% of the time and then makes you tear your hair out.)
>
> On Thu, Feb 23, 2017 at 9:56 PM, Harendra Kumar <harendra.kumar at gmail.com>
> wrote:
>
>> My first guess was a pattern match, but it sounded a bit odd because
>> there is no explicit constructor in case of numbers.  If there were an
>> explicit constructor it would have been easier to imagine this as a pattern
>> match. This seems to be a weird side effect of the special handling of
>> numbers.
>>
>> -harendra
>>
>> On 24 February 2017 at 07:37, Brandon Allbery <allbery.b at gmail.com>
>> wrote:
>>
>>> On Thu, Feb 23, 2017 at 9:05 PM, Harendra Kumar <
>>> harendra.kumar at gmail.com> wrote:
>>>
>>>> Kids have this amazing ability to break any toy in minutes. I gave my
>>>> seven year old daughter ghci to play with and in a little while she said it
>>>> is broken:
>>>>
>>>> >> let 1 = 2
>>>>
>>>> >> 1
>>>>
>>>> 1
>>>>
>>>> >>
>>>>
>>>> Earlier, I had explained to her about symbols and assigning values to
>>>> symbols, and I said numbers are not symbols. But when she came up with this
>>>> I could not explain what's going on. How can "1 = 2" be a valid equation?
>>>> Am I missing something fundamental here, or it is just broken?
>>>>
>>> It's a pattern match. The match fails, but as it produced no bindings it
>>> cannot be observed and its success or failure is irrelevant.
>>>
>>> --
>>> brandon s allbery kf8nh                               sine nomine
>>> associates
>>> allbery.b at gmail.com
>>> ballbery at sinenomine.net
>>> unix, openafs, kerberos, infrastructure, xmonad
>>> http://sinenomine.net
>>>
>>
>>
>
>
> --
> brandon s allbery kf8nh                               sine nomine
> associates
> allbery.b at gmail.com
> ballbery at sinenomine.net
> unix, openafs, kerberos, infrastructure, xmonad
> http://sinenomine.net
>
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