Re: [Haskell-cafe] Rewrite this imperative in FP way

Okay. But that's misleading, as normally x == y = True *iff* compare x y = EQ, but there this is not verified, as you don't redefined A function like "unionBy" doesn't exist, so IMHO to limit ambiguity, it would be a good idea to use MyTuple *only *at the specific place where you need its Eq instance instead of that of tuple (and then remove the Ord MyTuple instance), and use regular tuples elsewhere. The following should do the job: map getTuple . (`union` c) . map MyTuple 2012/2/6 Haisheng Wu

The reason I redefined Eq is: When doing `union` over two list of MyTuple is just base on its first element. Basically it means: [(1,2), (2,2)] `union` [(1,0), (2,0), (0,0)] produce [(1,2), (2,2), (0,0)] rather than [(1,2),(2,2),(1,0),(2,0),(0,0)] by default.

-Haisheng

On Sun, Feb 5, 2012 at 11:37 PM, Yves Parès wrote:

...

Concerning your first solution, I don't understand why you redefine Eq but not Ord instance. Ord will still work by comparing the tuples and not the first elements of said tuples. Plus the good news is you don't have to do this: just use regular tuples and use sort*By *or group*By *functions from Data.List with the 'on' function from Data.Function. For instance your Eq instance could have been written x == y = (==) `on` (fst . getTuple)

With regular tuples you can write "sortBy (compare `on` fst)".

Plus can you rewrite your original imperative algorithm with the right variable names? You're using a 'd' array that's not been defined.

2012/2/5 Haisheng Wu

...

a = [1,1,1,1] b = [0,1,2,3] d = [0,0,0,0]

for i in b: for j in c: if (i+j)<3: d[i+j] += a[i]

My just work implementation in Haskell http://hpaste.org/57452

Another people implementation in Haskell with Monad and it turns out complex and very imperatively. http://hpaste.org/57358

Do you have any cool solution in FP way?

Thanks. -Simon

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