The monomorphic package

[ Tags: bsd3, data, library ] [ Propose Tags ]

This library provides the type-class and functions to convert between polymorphic data-types and its monomorphic representation type, such as length-indexed vectors, singletons for type-level natural numbers, etc.

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Dependencies base (>=2.0 && <5), transformers (>=0.2 && <0.4) [details]
License BSD3
Copyright (C) Hiromi ISHII 2013
Author Hiromi ISHII
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Source repo head: git clone git://
Uploaded Fri Mar 15 07:39:31 UTC 2013 by HiromiIshii
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The monomorphic library

What is this?

This library provides the type-class and functions to convert polymorphic data-types to/from its monomorphic representation types. It is convenient to provide a monomorphic interface for dependently-typed programs.


Consider the following example:

data Nat = Z | S Nat
data SNat (n :: Nat) where
  SZ :: SNat Z
  SS :: SNat n -> SNat (S n)

instance Monomorphicable SNat where
  type MonomorphicRep SNat = Int
  promote 0     = Monomorphic SZ
  promote n
    | n < 0     = error "negative number"
    | otherwise =
        case promote (n - 1) of
          Monomorphic sn -> Monomorphic (SS sn)
  demote (Monomorphic sn) = toInt sn

data Vector (a :: *) (n :: Nat) where
  VNil :: Vector a Z
  (:-) :: a -> Vector a n -> Vector a (S n)

instance Monomorphicable (Vector a) whre
  type MonomorphicRep (Vector a) = [a]
  demote  (Monomorphic n) = toList n
  promote [] = Monomorphic Nil
  promote (x:xs) =
    case promote xs of
      Monomorphic xs' -> Monomorphic $ x :- xs'

Monomorphic k is the wrapper type to eliminate the polymorphic part of the types. The Monomorphicable type-class provides the functions to convert polymorphic value from/to its monomorphic representation, say MonomorphicRep k associated type.

In the example above, SNat n can be monomorphically represented by Int. There are some convenient functions to manipulate monomorphic types and functions. For more detail, see API documentation.