Hi, I have some questions about using the empty list in Hugs. In June 2001, this was raised as a bug in hugs (see Hugs-Bugs Archives). References therein to a bug list on S. Thompson's pages come up a dead end. I haven't found any follow up or solution to this discussed. I would particularly like to use an empty list in the Trex module of Hugs. Suppose I have two expressions: emptyListA = null emptyListB = [] emptyListA is apparently a function from empty lists to Bool. I would have thought that emptyListB would just be the empty list and output it as such.
:t emptyListA
emptyListA :: [a] -> Bool
emptyListA [] True
The problem is that there is no "show" function for emptyListB, which is just []
emptyListB ERROR - Cannot find "show" function for: *** Expression : emptyListB *** Of type : [a]
What I would like, simply is:
emptyListB []
More to my purpose, I would like to use the empty list as a value of a record in Trex. For instance, while the first example below gives a good result, the second example has no "show" function. Looking at the Trex module, it would seem that there is a show function for the empty list (?? but frankly, I have never touched this area of haskell, so I haven't given it much thought). Sample command line I/O.
(a = 'a', b = [2]) (a = 'a', b = [2])
(a = 'a', b = []) ERROR - Cannot find "show" function for: *** Expression : (a = 'a', b = []) *** Of type : Rec (a :: Char, b :: [a])
Clearly, the problem is the empty list labelled b. What I want is the following sort of result:
(a = 'a', b = []) (a = 'a', b = [])
Further along, what I want is to have values for the field labelled b to be lists which contain something. So, in one case, the value is [], while in another it is [2], say. Moreover, I want to define a type which has a label with values of type lists. That is, I want to have a record something like Rec (a :: Char, b :: List). However, List is not a basic data type. And in any case, the values of labels cannot show empty lists. I'm unsure whether Trex or any record type can handle this. Here are some examples. The following is OK. type TestList01 = Rec (a :: Char, b :: [Char]) testList01 :: TestList01 testList01 = (a = 'a', b = "b") Input and output are:
testList01 (a = 'a', b = "b")
ComplexActions01> :t (a = 'a', b = "b") (a = 'a', b = "b") :: Rec (a :: Char, b :: [Char]) But the following type is uninterpretable: type TestList02 = Rec (a :: Char, b :: []) ERROR "ComplexActions01.lhs":196 - Illegal type "[]" in constructor application. Suggestions about how to treat empty lists? Thanks, Adam Wyner