T is the type. A and B are the only constructors for values of that type. A and B are not terms in the type language. T is not a term in the value language.

It's simpler to consider a type without any fields in the constructor:

data Bool = True | False

True and False are values, Bool is the type. You can't use Bool as a constructor, and you can't use True or False as a type.

When you add fields it can get a bit more confusing, because the fields of a constructor are types, so it looks like "ValueConstructor1 FieldType1 FieldType2 | ValueConstructor2 FieldType3"

data PersonOrPlace = Person String | Place String

To make it more clear, here the types are annotated with <AngleBrackets> and the constructors annotated with [SquareBrackets]:

data <PersonOrPlace> = [Person] <String> | [Place] <String>

On Tue, Jun 16, 2015 at 8:52 AM, Matt Williams <matt.williams45.mw@gmail.com> wrote:

Dear All,

I am sure this is a common mistake, and I am happy to be pointed elsewhere for reading.

I have spent the last couple of days on the Haskell irc channel, which was very helpful.

However, one of the points of discussion left me confused.

When we have a type, T, with constructors A and B

(e.g. data T = A x y z | B x y)

How do I understand the relationship between A, B and T? I had thought I could use the sub-class relationship, but that doesn't seem to be true.

Any other pointers very welcome.

Matt

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