Re: [Haskell-cafe] What is an "expected type" ...
When Haskell runs it's type checker, it tries to "guess" the type of each function. Thats why you can write: map (+1) and it knows that you're talking about a function of type: Num a => [a] -> [a] Another thing, called 'defaulting' resolves this, but you didn't ask about that, so I won't go into it. An expected type is one that you provide to the compiler in the form of a type signature, this can be used to specialize a general type (like the one I showed) or to resolve ambiguous types the compiler can't, or just for documentation/good practice. So when I write: foo :: Num a => [a] -> [a] foo ls = map (+1) ls The "expected type" for `foo` is `Num a => [a] -> [a]`. I imagine you're asking this because you got an error which said your expected type doesn't match your inferred type. That might, for instance, happen if I wrote: bar :: String bar = 'a' 'a' has type `Char`, since `String` is not `Char`, the type checker infers that 'a' has type char, but _expects_ it to be type String. Two solutions are as follows: --- Method 1 bar :: Char bar = 'a' --- Method 2 bar :: String bar = "a" Can you see why those two changes fix the problem? Also, just as a matter of process, I forwarded this to the haskell-beginners list, as I imagine type errors like these come up a lot, and someone probably has a better explanation over there. /Joe michael rice wrote:
as opposed to an "inferred type"?
Michael
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Hey Joe, all,
Got it. Thanks!
An associated question: In programming a local "helper" or "auxilliary" function such as dec2bin' in
dec2bin :: Integer -> [Integer]
dec2bin n = dec2bin' n []
where dec2bin' n acc
| n == 0 = acc
| otherwise = let r = rem n 2
m = div (n - r) 2
in dec2bin' m (r : acc)
is there any way to assign a type signature to the helper function?
Michael
--- On Sun, 6/28/09, Joe Fredette
as opposed to an "inferred type"?
Michael
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On Jun 28, 2009, at 12:02 , michael rice wrote:
dec2bin :: Integer -> [Integer] dec2bin n = dec2bin' n [] where dec2bin' n acc | n == 0 = acc | otherwise = let r = rem n 2 m = div (n - r) 2 in dec2bin' m (r : acc)
is there any way to assign a type signature to the helper function?
Same way you do for a top level binding:
dec2bin :: Integer -> [Integer] dec2bin n = dec2bin' n [] where dec2bin' :: Integer -> [Integer] -> [Integer] dec2bin' n acc | n == 0 = acc | otherwise = let r = rem n 2 m = div (n - r) 2 in dec2bin' m (r : acc)
-- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH
How else? ;-)
Thanks,
Michael
--- On Sun, 6/28/09, Brandon S. Allbery KF8NH
Am Sonntag 28 Juni 2009 18:06:52 schrieb Brandon S. Allbery KF8NH:
On Jun 28, 2009, at 12:02 , michael rice wrote:
dec2bin :: Integer -> [Integer] dec2bin n = dec2bin' n [] where dec2bin' n acc
| n == 0 = acc | otherwise = let r = rem n 2
m = div (n - r) 2 in dec2bin' m (r : acc)
is there any way to assign a type signature to the helper function?
Same way you do for a top level binding:
dec2bin :: Integer -> [Integer] dec2bin n = dec2bin' n [] where dec2bin' :: Integer -> [Integer] -> [Integer] dec2bin' n acc
| n == 0 = acc | otherwise = let r = rem n 2
m = div (n - r) 2 in dec2bin' m (r : acc)
But, to mention it before it bites, putting type signatures involving type variables on local helper functions is not entirely straightforward. Consider inBase :: Integral a => a -> a -> [a] 0 `inBase` b = [0] n `inBase` b = local n [] where local 0 acc = acc local m acc = case m `divMod` b of (q,r) -> local q (r:acc) Now try giving a type signature to local. You can't. What is the type of local? It's (type of b) -> [type of b] -> [type of b], but "type of b" isn't available. If you try local :: a -> [a] -> [a] or local :: Integral a => a -> [a] -> [a], you are saying that local works for *every* type a (or for every type a which is an instance of Integral), because the 'a' from local's type signature is a new (implicitly forall'd) type variable. To be able to give local a type signature, you must bring the type variable 'a' into scope: {-# LANGUAGE ScopedTypeVariables #-} inBase :: forall a. Integral a => a -> a -> [a] 0 `inBase` b = [0] n `inBase` b = local n [] where local :: a -> [a] -> [a] -- now this a is the same a as the one above local 0 acc = acc local m acc = case m `divMod` b of (q,r) -> local q (r:acc)
participants (4)
-
Brandon S. Allbery KF8NH -
Daniel Fischer -
Joe Fredette -
michael rice