[Haskell-beginners] Difference between types and values

[Bob Ippolito]

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 at 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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