Hi Julian,
Check out my package ctrex. https://www.haskell.org/haskellwiki/CTRex
Cheers,
Atze
On Dec 30, 2014 6:20 PM, "Julian Arni"
Hi all,
I've been playing around with what might be described as type-directed functions. One example is a list-like structure of phantom-typed values
{-# LANGUAGE TypeOperators #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-}
import GHC.TypeLits
infixr 6 ::: data a ::: b = a ::: b deriving (Show, Eq)
data Tag b = Tag String deriving (Show, Eq)
ex1 :: Tag 5 ::: Tag 3 ::: Tag 7 ex1 = Tag "Alice" ::: Tag "Bob" ::: Tag "Carol"
And then sorting 'ex1' based on the Nats, such that
sort ex1 :: Tag 3 ::: Tag 5 ::: Tag 7 sort ex1 = Tag "Bob" ::: Tag "Alice" ::: Tag "Carol"
Notice how it's the types, not the values, that determine the result, but that the value-level also changes.
I know how to do this using classes, but it's a little excruciating - it's like programming in a verbose and very restricted Prolog. With type families it's much easier to get the result *type* (pattern matching is simple, recursive calls are natural, and it all looks a lot more like Haskell), but I haven't yet seen a way of effectively using type families to direct the value-level component of the calculation.
Are there any examples of how this might be done? Or are there other alternatives to using type-classes that I am missing? Or, alternatively, are there libraries to reduce the boilerplate of this type-class code?
Thanks, Julian
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