Hello, I imagine people wanting to do things as in the example below. If we were to use only `TypeError` constraints, then we'd have to mostly use the class system to do type-level evaluation. It doesn't seem obvious how to do that with just `TypeError` of kind constraint, unless all evaluation was to happen using the class system.
-Iavor PS: Interestingly, this example reveals a bug with GHC's new warning about unused constraints, where the `OffsetOf` constant on `get` is reported as unnecessary... I'll file a bug. {-# LANGUAGE TypeFamilies, TypeOperators, UndecidableInstances, DataKinds #-} {-# LANGUAGE ScopedTypeVariables #-} import GHC.TypeLits import Data.Proxy import Data.Word import Foreign.Ptr type OffsetOf l xs = GetOffset 0 l xs type family ByteSize x where ByteSize Word64 = 8 ByteSize Word32 = 4 ByteSize Word16 = 2 ByteSize Word8 = 1 ByteSize a = TypeError (Text "The type " :<>: ShowType a :<>: Text " is not exportable.") type family GetOffset n (l :: Symbol) xs where GetOffset n l ( '(l,a) ': xs) = '(n,a) GetOffset n l ( '(x,a) : xs) = GetOffset (n+ByteSize a) l xs GetOffset n l '[] = TypeError (Text "Missing field: " :<>: ShowType l) newtype Struct (a :: [(Symbol,*)]) = Struct (Ptr ()) get :: forall l fs n a. (OffsetOf l fs ~ '(n,a), KnownNat n) => Struct fs -> Proxy l -> Ptr a get (Struct p) _ = plusPtr p (fromInteger (natVal (Proxy :: Proxy n))) type MyStruct = [ '("A",Word8), '("B",Word8), '("C",Int) ] testOk :: Struct MyStruct -> Ptr Word8 testOk s = get s (Proxy :: Proxy "B") {- testNotOk :: Struct MyStruct -> Ptr Word8 testNotOk s = get s (Proxy :: Proxy "X") --} {- type MyOtherStruct = [ '("A",Int), '("B",Word8) ] testNotOk :: Struct MyOtherStruct -> Ptr Word8 testNotOk s = get s (Proxy :: Proxy "B") --} On Mon, Nov 16, 2015 at 1:21 PM, Ben Gamari <b...@smart-cactus.org> wrote: > > While preparing some additional documentation for Iavor's custom type > errors work (which has been merged; thanks Iavor!) I noticed that > Dominique Devriese has raised some additional questions on the proposal > [1]. > > In particular, Dominique suggests that perhaps TypeError should simply > be of kind `ErrorMessage -> Constraint`. My understanding of the > proposal is that the intention is that `TypeError`s should be usable on > the RHS of type functions, like `error` on the term level. However, is > this strictly necessary? Is there any place where you couldn't just as > easily express the `TypeError` as a constraint? > > If not, it seems like this may be substantially simpler approach and > totally side-steps the detection of `TypeError` in inappropriate places > on the RHS. > > Regardless, it seems like this (and the other questions) is worth > addressing in the proposal. > > Cheers, > > - Ben > > > [1] > https://ghc.haskell.org/trac/ghc/wiki/Proposal/CustomTypeErrors#SomedesignquestionsDominiqueDevriese > : >
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