# [Haskell-cafe] Functions that return functions

Sun Apr 12 23:59:21 EDT 2009

```On Apr 12, 2009, at 11:50 PM, michael rice wrote:

> I'm not sure what you're trying to tell me, so let's try a specific
> case.
>
>
> Here's the makeVerifier function that expects its function f to have
> a single arg, a pair (i,d):
>
> makeVerifier :: ((Int,Int) -> Int) -> Int -> (Int -> Bool)
> makeVerifier f m = divides m . foldl (+) 0 . map f . zip [1 .. ] .
> digits
>
> And usage:
> let checkCc = makeVerifier (\ (i d) -> if odd i then d else if d < 5
> then 2*d else 2*d + 1) 10
>

This looks like it has a typo -- did you mean \ (i, d) -> rather than
\ (i d) -> ?

>
>
> And here's the old makeVerifier function that expects its function f
> to have two integer arguments, i & d:
>
> makeVerifier :: (Int -> Int -> Int) -> Int -> (Int -> Bool)
> makeVerifier f m = divides m . foldl (+) 0 . zipWith f [1 .. ] .
> digits
>
> And usage:
> let checkCc = makeVerifier (\ ....    <== Complete this  ) 10
>

let checkCc = makeVerifier (\ i d -> if odd i then d else if d < 5
then 2*d else 2*d + 1) 10

though I find it a bit nicer to expand it a bit for clarity:

checkCc :: Int -> Bool
checkCc = makeVerifier f 10
where
f i d | odd i     = d
| d < 5     = 2*d
| otherwise = 2*d + 1

-Ross

>
> --- On Sun, 4/12/09, Ross Mellgren <rmm-haskell at z.odi.ac> wrote:
>
> From: Ross Mellgren <rmm-haskell at z.odi.ac>
> Subject: Re: [Haskell-cafe] Functions that return functions
> To: "michael rice" <nowgate at yahoo.com>
> Date: Sunday, April 12, 2009, 9:23 PM
>
> Under the covers of syntax they only have one parameter, but you can
> write curried lambdas or functions easily:
>
> \ a b -> a + b
>
> which is equivalent to
>
> \ a -> \ b -> a + b
>
> and also equivalent to the "normal" function syntax
>
> f a b = a + b
>
> or
>
> f a = \ b -> a + b
>
> -Ross
>
> On Apr 12, 2009, at 9:09 PM, michael rice wrote:
>
>> My question was meant in the context of the makeVerifier function,
>> which is passed a lambda expression. It's my understanding that
>> Haskell lambda expressions can have only a single parameter, which
>> is why I changed the function parameter to a pair, (i,d).
>>
>> How would it be done otherwise?
>>
>> Michael
>>
>>
>> --- On Sun, 4/12/09, Daniel Fischer <daniel.is.fischer at web.de> wrote:
>>
>> From: Daniel Fischer <daniel.is.fischer at web.de>
>> Subject: Re: [Haskell-cafe] Functions that return functions
>> To: "michael rice" <nowgate at yahoo.com>
>> Date: Sunday, April 12, 2009, 7:20 PM
>>
>> Am Montag 13 April 2009 01:09:22 schrieb michael rice:
>> >
>> > Michael
>> >
>>
>> Curried:
>>
>> f :: a -> b -> c
>>
>> amenable to partial application.
>>
>> Uncurried:
>>
>> g :: (a,b) -> c
>>
>> not easy to apply partially.
>>
>> The Prelude contains
>>
>> curry :: ((a,b) -> c) -> (a -> b -> c)
>>
>> uncurry :: (a -> b -> c) -> ((a,b) -> c)
>>
>> to convert if needed.
>>
>> _______________________________________________