Synonym to constrained type

Question

I know I can create synonym for a type, for example:

type ListToArray a e = [e] -> a Int e

I also read that a constrained type is still a type, so I thought I can create a synonym for that too:

type (Data.Array.IArray.IArray a e) => ListToArray a e = [e] -> a Int e

However GHC complains:

parse error on input '=>'

Does GHC support synonyms to constrained types?

Answer 1

You can check the Haskell 2010 Report, the section about type synonyms states that the grammar for type synonyms is:

type simpletype = type
simpletype  →   tycon tyvar1 … tyvark       (k ≥ 0) 

In other words: no you cannot put class constraints on the left of the = in a type declaration.

Also note that you cannot have class constraints even on the right of the = in such declarations. The definition of type is:

type    →   btype [-> type]         (function type)

btype   →   [btype] atype       (type application)

atype   →   gtycon
    |   tyvar
    |   ( type1 , … , typek )       (tuple type, k ≥ 2)
    |   [ type ]        (list type)
    |   ( type )        (parenthesised constructor)

gtycon  →   qtycon
    |   ()      (unit type)
    |   []      (list constructor)
    |   (->)        (function constructor)
    |   (,{,})      (tupling constructors) 

So the type non-terminal refers to a type without a context. In other words: no you cannot have a context in a type synonym, at least not in the general case.

GHC has a lot of extension to Haskell's type system that can lift some of the above restrictions in certain situations (see this page.) however, AFAIK, none that achieves exactly what you want.