[Haskell-cafe] Mild confusion around type family

Nicholls, Mark nicholls.mark at vimn.com
Sun Jun 21 09:29:28 UTC 2015

Let me have a go with both of these

Excuse the spelling, sent from a phone with itty bitty keys, it like trying to sow a button on a shirt with a sausage.

On 20 Jun 2015, at 16:35, adam vogt <vogt.adam at gmail.com<mailto:vogt.adam at gmail.com>> wrote:

On Jun 19, 2015 2:29 PM, "Richard Eisenberg" <eir at cis.upenn.edu<mailto:eir at cis.upenn.edu>> wrote:
> With a closed family, you could always do something like this:
> > type family Error (msg :: Symbol)     -- no instances!
> > type family Sub a b where
> >   -- real work
> >   Sub a b = Error "Sub is undefined here"
> Would that help? It still doesn't immediately error (there's no way to do that currently), but it's perhaps more telling to have an error string.

Ghc does error out right away if you give it an infinite loop:

type family Fail (x::k) (a :: *) where
  Fail x a = Fail x (Fail "don't raise the context stack please" a)

-- defining f leads to compile failure, and the message includes the "msg"
f :: Fail "msg" ()
f = undefined

But it would be nice to have a less hacky solution, especially as this one gives error messages that suggest giving the type checker more resources.


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