[clean-list] understanding list type definition

[Rinus Plasmeijer]

Dear Jigang

Not every error can be covered by syntax rules.

In principle expression like these are allowed.

For instance if you define

:: List a = Cons a (List a) | Nil

then List Bool is a legal type as well as [List Bool]

But, indeed [Int Bool] is not.
The syntax is not wrong, but there is a semantic restriction that a common 
type should be of kind *. One cannot apply Int to Bool because Int is of 
kind *.
One can apply List to Bool because List is of kind (* -> *) and Bool is of 
kind *
such that List Bool is of kind *.
So, shortly, [Int Bool] is syntactically ok, but semantically there is a 
type error.

I hope this clarifies things,

Greetings,

Rinus




----- Original Message ----- 
From: "Jigang Sun" <Jigang.Sun at student.paisley.ac.uk>
To: <clean-list at science.ru.nl>
Sent: Wednesday, October 04, 2006 10:42 AM
Subject: [clean-list] understanding list type definition


> On language report 2.0, page 115,
>
> ListType = [[ListKind] Type [SpineStrictness]]
> Type = {BrackType}+
> BrackType = [UniversalQuantVariables] [Strict] [UnqTypeAttrib] 
> TypeExpression
>
> Type Expression = TypeVariable // see A.7
> |typeConstructorName
> | (Type)
> | PredefinedType
> | PredefinedTypeConstructor
>
> PredefinedType = BasicType
> | ListType
> | TupleType
> | ArrayType
> | ArrowType
> | PredefType
>
> BasicType = Int
> | Real
> | Char
> | Bool
>
> so a list type [Int Bool] is legal?
>
> Jigang
>
>
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