Dear Benjamin,
I'm not sure I quite get your problem. My guess: you want a type-level
guarantee that `turnOff'` function will only be applied to the terms of
type `State True`, and so, you expect a compile-time error from `turnOff'
off`. Am I get it right?
In your solution you seem to over engineer the solution. You try to relate
type-level informaton (the value of of the type parameter `s` of `State`)
to the term-level one (the value of the field stored in the `State`
datatype). I think you'd better read how this task is solved with the
technique known as singletons (
https://cs.brynmawr.edu/~rae/papers/2012/singletons/paper.pdf).
But I bet you don't need to solve that task to just address the problem at
hand, if I understand the problem correctly. Please, tell me if the simpler
solution below suits you: it doesn't use term-level (field of State)
information at all.
{-# LANGUAGE DataKinds, TypeFamilies #-}
data State (s :: Bool) = State deriving Show
off :: State False
off = State
type family TurnOff s where
TurnOff True = False
turnOff :: State True -> State False
turnOff State = State
bad = turnOff off -- <-- error: Couldn't match type ‘'False’ with ‘'True’
main = print bad
https://ideone.com/boWN1q
--
Best, Artem
On Fri, 22 Jun 2018 at 12:46 Benjamin Franksen
I have observed that type functions seem to be lazy. A type function that is partial does not give me a type error when it is called with an argument for which it is not defined.
Is there a 'seq' at the type level?
Here is my use case, simplified of course. Suppose we want to statically track the state of a switch with a phantom type parameter:
{-# LANGUAGE DataKinds, TypeFamilies #-}
data State (s :: Bool) = State Bool deriving Show
off :: State False off = State False
We want user code to be able to call the function 'turnOff' only if the switch is currently on:
turnOff :: State True -> State False turnOff (State True) = State False
This works fine:
*Main> turnOff off
<interactive>:1:9: error: ? Couldn't match type ?'False? with ?'True? Expected type: State 'True Actual type: State 'False
But now I want to abstract this pattern and write a (closed) type function
type family TurnOff s where TurnOff True = False
turnOff' :: State x -> State (TurnOff x) turnOff' (State True) = State False
bad = turnOff' off
*Main> :t bad bad :: State (TurnOff 'False) *Main> bad *** Exception: TypeLevelSeq.lhs:37:3-37: Non-exhaustive patterns in function turnOff'
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