Editing Type (section)

==Type and newtype==

The other two ways one may introduce types to Haskell programs are via the <hask>type</hask> and <hask>newtype</hask> statements.

<hask>type</hask> introduces a synonym for a type and uses the same data constructors. <hask>newtype</hask> introduces a renaming of a type and requires you to provide new constructors.

When using a <hask>type</hask> declaration, the type synonym and its base type are interchangeble almost everywhere (There are some restrictions when dealing with [[instance]] declarations). For example, if you had the declaration:

<haskell>
 type Name = String
</haskell>

then any  [[function]] you had declared that had <hask>String</hask> in its signature could be used on any element of type <code>Name</code>

However, if one had the declaration:

<haskell> 
  newtype FirstName = FirstName String
</haskell>

this would no longer be the case. Functions would have to be declared that actually were defined on '''FirstName'''. Often, one creates a deconstructor at the same time which helps alleviate this requirement. e.g.:
<haskell>
  unFirstName :: FirstName -> String
  unFirstName (FirstName s) = s
</haskell>
This is often done by the use of  [[field|field labels]] in the <code>newtype</code>. (Note that many consider the Haskell field implementation sub-optimal, while others use it extensively. See [[Programming guidelines#Records|Programming guidelines]] and [[Future of Haskell]])