Re: [Haskell-cafe] strange error message

Hi, It seems to be already fixed: https://ghc.haskell.org/trac/ghc/changeset/b988dc39a06278acc2373ba9a40ee08da... Thanks, Yuras On Thu, 2014-03-27 at 13:16 +0100, Erik Hesselink wrote:

Could you create a code snippet that reproduces the error?

Erik

On Thu, Mar 27, 2014 at 1:09 PM, Semen Trygubenko / Семен Тригубенко wrote:

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Hi Ivan,

On Thu, Mar 27, 2014 at 11:28:33AM +0000, Ivan Perez wrote:

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What is the type of head? What is the type of length? What is the type of length.head? What is the type of r?

Prelude> :t head head :: [a] -> a Prelude> :t length length :: [a] -> Int Prelude> :t (length . head) (length . head) :: [[a]] -> Int Prelude>

r is of type Int.

[Apologies — I thought the above is deducable from the error message I've included (below).]

Clearly, this is a type error as Int is passed in where [[a]] is expected. However, my question is relating to the middle bit of the error message, namely:

(The function `length . head' is applied to one argument, but its type `[[a2]] -> Int' has only one)

Many thanks, S.

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On 27 March 2014 02:36, Semen Trygubenko / Семен Тригубенко < semen@trygub.com> wrote:

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Dear Haskell-Cafe,

Every now and then I get error messages when compiling Haskell code :). Sometimes of the sort:

test.hs:10:15: Couldn't match type `[[a2]]' with `Int' Expected type: Int -> Int Actual type: [[a2]] -> Int The function `length . head' is applied to one argument, but its type `[[a2]] -> Int' has only one In the first argument of `(==)', namely `(length . head) r' In the expression: (length . head) r == 0

Is the middle part of the error message

(The function `length . head' is applied to one argument, but its type `[[a2]] -> Int' has only one)

expected to be confusing like that sometimes?

The line in question, FWIW, is

Right r -> if (length . head) r == 0

It appears as one of the two entries in a case statement, pattern matching on Right constructor.

Many thanks, S.

-- Семен Тригубенко http://trygub.com

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-- Семен Тригубенко http://trygub.com

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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