# [Haskell-cafe] Pattern matching on numbers?

Tobias Bexelius tobias.bexelius at avalanchestudios.se
Wed Nov 19 06:01:03 EST 2008

```Yes, fromInteger and == is used for pattern matching on numbers.
However, on GHC you can use -XNoImplicitPrelude to make it use whatever
fromInteger and == that's in scope (without any Num or Eq).

Eg. if == is a method of MyEq class and fromInteger is a method of
MyNum, and MyNum doesn't inherit MyEq, then the type of this function

f 0 = ""
f x = 'e' : f (x-1)

Will be inferred as (MyEq a, MyNum a) => a -> String

/Tobias

-----Original Message-----
Sent: den 19 november 2008 01:00
To: Henning Thielemann
Subject: Re: [Haskell-cafe] Pattern matching on numbers?

On Tue, Nov 18, 2008 at 6:56 PM, Henning Thielemann
<lemming at henning-thielemann.de> wrote:
>
> On Tue, 18 Nov 2008, Ryan Ingram wrote:
>
>> How does this work?
>>
>>> fac n = case n of
>>>   0 -> 1
>>>   _ -> n * fac (n-1)
>>
>> ghci> :t fac
>> fac :: (Num t) => t -> t
>>
>> The first line of "fac" pattern matches on 0.  So how does this work
>> over any value of the Num typeclass?  I know that the "1" on the rhs
>> of fac are replaced with (fromInteger 1), but what about numeric
>> literals in patterns?  Does it turn into a call to (==)?
>
> As far as I know, yes. It is even possible to trap into an error on
> pattern matching this way if fromInteger generates an 'undefined'.

As I understand it, the use of (==) in numeric pattern matching is why
Num requires Eq.

--
Dave Menendez <dave at zednenem.com>
<http://www.eyrie.org/~zednenem/>
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