[Haskell-cafe] Of types and constructors; a question of programming style

6 Jul 2004

      When I'm designing datatypes for a Haskell program, I sometimes seem to end 
up with a slightly incoherent mixture of algebraic types and 
constructors.  I'm wondering if anyone has worked out a principled way to 
decide when to introduce a new data type.

Here's an example from something I'm working on:

[[
data Event = Document DocURI Element
            | Element Name BaseURI Language Children Attributes LiIndex Subject
            | EndElement
            | Attribute Name AttributeVal
            | Text TextVal
]]
(abbreviated from the original for discussion, and using arbitrary type 
names for the field types)

Using this formulation, I have no access to the type of the different 
alternatives of the Union type thus defined.  A different way of describing 
the type overcomes this limitation:
[[
data Event = EVDocument Document
            | EVElement Element
            | EVEndElement EndElement
            | EVAttribute Attribute
            | EVText Text

data Document = Document DocURI Element

data Element = Element Name BaseURI Language Children Attributes
                        LiIndex Subject

data EndElement = EndElement

data Attribute = Attribute Name AttributeVal

data Text = Text TextVal
]]
which seems to be rather repetitious.  And I haven't even started to use 
the named field syntax here.

Yet another alternative, if I'm not interested in field names, might be to 
use tuples and type synonyms for the alternative values:
[[
data Event = Document EVDocument
            | Element EVElement
            | EndElement EVEndElement
            | Attribute EVAttribute
            | Text EVText

type EVDocument = (DocURI,Element)

type EVElement = (Name,BaseURI,Language,Children,Attributes,LiIndex,Subject)

type EVEndElement = ()

type EVAttribute = (Name,AttributeVal)

type EVText = TextVal
]]

Is there any community consensus concerning what constitutes good style in 
such circumstances?

#g

------------
Graham Klyne
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http://www.ninebynine.org/#Contact