Monomorphize polymorphic functions for testing
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Test Polymorphic Functions with Metamorph

I give you an unknown function:

f :: forall a. a -> (a -> a) -> a

By looking at its type, you know it can do only one thing: iterate the second argument on the first one a fixed number of times. You can easily extract that number out of the function by instantiating f at the type of integers, and by applying it to 0 and succ.

> f 0 succ :: Integer

From the type of f and a single value, you can deduce that f just iterates a function three times:

f :: forall a. a -> (a -> a) -> a
f a r = r (r (r a))

Any polymorphic function is similarly restrained by its type in the range of possible behaviors it can have, and we can restrict it further by observing some of its outputs.

Given a universally quantified type, e.g., forall a. a -> (a -> a) -> a, metamorph can craft a type A and special inputs to perform a sort of symbolic evaluation of functions of that type.

Reverse engineer functions

Functions of certain types, such as forall a. a -> (a -> a) -> a, are uniquely determined by the image of a single well chosen input. In that case, we can guess the definition of a polymorphic function with prettyMorphing:

prettyMorphing :: (...) => String -> monomorphizedType -> String
prettyMorphing :: String -> (A -> (A -> A) -> A) -> String`

> prettyMorphing "f" (f :: A -> (A -> A) -> A)
"f a b = b (b (b a))"

Random evaluation

In other cases, prettyMorphing won't type check. You may try to use morphingGen to generate some random inputs. Thanks to QuickCheck, even functions can be generated!

For any function type F a and an associated A constructed by metamorph:

morphingGen :: F A -> Test.QuickCheck.Gen (Domain (F A), Codomain (F A))

For example, if F a = [a] -> [a], then Domain (F A) ~ ([A], ()) and Codomain (F A) ~ [A].

> generate (morphingGen (reverse :: [A] -> [A])) >>= \((a, _), c) ->
>   putStrLn $ "a@" ++ pretty_ a ++ " -> " ++ show c
a@[_,_,_,_] -> [a !! 3,a !! 2,a !! 1,a !! 0]

Some list a of length 4 was generated, and this shows that the image of a under reverse is indeed its reverse.

We use pretty_ instead of show to display the input list with minimal noise.

Examples can be found under test/.

-- No Template Haskell! Magic!

{-# LANGUAGE TypeFamilies #-}

import Data.Functor.Identity
import Test.Metamorph

type F a = a -> (a -> a) -> a

-- This is our polymorphic function.
f :: {- forall a. -} F a
f a r = r (r (r a))

-- The only ingredient you need to invent is the type synonym F.
-- Everything else (Metamorph, Newtype) belongs to Test.Metamorph.
-- The first incantation is one newtype and a trivial (enough) instance
-- to unwrap it.

newtype A' = A' (F (Metamorph A'))

instance Newtype A' where
  type Old A' = F (Metamorph A')
  unwrap (A' a) = a

-- This is the type at which to instantiate f.
-- It is Show-able.
type A = Metamorph A'

-- Monomorphization!
f_ :: A -> (A -> A) -> A
f_ = f

-- For this type, you only need to look at a single value to deduce everything
-- about f. metamorph implicitly and purely computes an ideal pair (A, A -> A)
-- to be applied to f.
-- 'morphingPure' applies these arguments to the function, and returns the
-- result.
-- > morphingPure :: F A -> A
-- 'prettyMorphing' produces a possible definition of the function.
-- > prettyMorphing :: String -> F A -> String
main = do
  print (morphingPure f_)            -- b (b (b a))
  putStrLn (prettyMorphing "f" f_)   -- f a b = b (b (b a))

Future work

  • Documentation.

  • We could also enumerate inputs. Even if there may be more than one necessary value to observe, there are many cases where they live in a very small space; e.g., forall a. [a] -> [a], only one list of each length is necessary to identify a function of that type.

  • Generic implementation for algebraic data types. The type F must currently use only:

    • (), Bool, Integer, Int, (->), (,), Either, Maybe, [].


If you have two or more type parameters:

type G2 a b = (a -> b -> b) -> [a] -> b -> b

Merging them will do the right thing:

type G a = G2 a a


  • Testing polymorphic properties. Jean-Philippe Bernardy, Patrik Jansson, and Koen Claessen. 2010.
    In Proceedings of the 19th European conference on Programming Languages and Systems (ESOP'10), Andrew D. Gordon (Ed.). Springer-Verlag, Berlin, Heidelberg, 125-144.

The main idea comes from this paper: we can construct a "good" type to test polymorphic functions on. The given construction is decomposed into simple operations on types, which works well for proofs, but that seems tough to implement in Haskell.

metamorph takes a more streamlined approach centered on the notion of symbolic evaluation. Interestingly, it turns out that GHC's type families suffice to implement this without using Template Haskell.