I thought I had already replied to this, but I must have forgotten to get back and post it.
I dislike this solution because it's forcing people into doing things they don't want to do. You don't need to provide all three; view, zoom and over -- that's un-necessary because if people need those they can just use the lenses, but you do need to provide a "complete" API so that someone can actually build a simple program with the option to extend it later with lenses when they feel more comfortable. This means for example, `view decoded` is an important skeleton to provide and so is `encoder`. Adding these debugging functions may fix the problem, but the solution is not the correct one. It's putting a lot of pressure on the user to feel that these errors are as a fault of them not being able to deal with the types produced by the libraries, rather than the more correct opinion that the libraries have made lens (an extension for seasoned users) the sole API, which (IMO) is wrong. Regards. On Tuesday, February 11, 2014 4:42:42 PM UTC+11, Gabriel Gonzalez wrote: > > Ok, I think I found an approach that will satisfy everybody. This > solution only requires providing three utility functions in `pipes-parse`: > > debugView > :: (Producer a m x -> Producer b m y) > -> (Producer a m x -> Producer b m y) > debugView = id > > debugZoom > :: (StateT (Producer a m x) m r -> StateT (Producer b m y) m r) > -> (StateT (Producer a m x) m r -> StateT (Producer b m y) m r) > debugZoom = id > > debugOver > :: ( (Producer a m w -> Producer b m x) > -> (Producer c m y -> Producer d m z) ) > -> ( (Producer a m w -> Producer b m x) > -> (Producer c m y -> Producer d m z) ) > debugOver = id > > The following example shows how you would use it (see the `p3` example): > > http://lpaste.net/99760 > > Notice how the type error is actually even *better* now than the > "intermediate function" proposal. > > There also need to be equivalent type-restriction functions for `FreeT` > transformations as well. Those will go in `pipes-group`. > > The nice advantages of this solution are: > > * There's no longer a need to provide three extra functions for every lens > (i.e. one each for `view`/`zoom`/`over`) > > * The user doesn't need to know what the restricted type should be. The > utility functions take care of that. > > * It's very easy to undo once the user has found the correct type, by > deleting the `debugXXX` function. This leaves the user with idiomatic > lens-based code. > > On 02/10/2014 02:51 PM, Kyle Van Berendonck wrote: > > I don't think teaching users how to "hack around" the titanic type error > issue is really a solution though, and certainly not in pipes-parse because > people who are interested in a particular pipes library are less likely to > read it there. > > On Monday, February 10, 2014 3:28:13 AM UTC+11, Gabriel Gonzalez wrote: >> >> So I was thinking some more about this, and I was wondering if perhaps >> the solution would be to simply explain in the `pipes-parse` tutorial how >> to simplify type errors using the trick of defining an intermediate >> non-lens function. >> >> The tutorial would say (in more words) that if you have a type error with >> a `lens` of type `Lens (Producer a m x) (Producer b m y)` while using >> `view`, then you should convert it to an intermediate ordinary function >> like this: >> >> intermediate :: Producer a m x -> Producer b m y >> intermediate = view lens >> >> ... then use that `intermediate` function for better type error >> inference. Similarly, if you have a type error using the `lens` with a >> `zoom`, then you convert it to the following intermediate ordinary function: >> >> intermediate:: StateT (Producer b m y) m r -> StateT (Producer a m x) >> m r >> intermediate = zoom lens >> >> Teaching beginners how to do this would help them improve type errors >> without having to add redundant functions to the API. >> > -- > You received this message because you are subscribed to the Google Groups > "Haskell Pipes" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] <javascript:>. > To post to this group, send email to [email protected]<javascript:> > . > > > -- You received this message because you are subscribed to the Google Groups "Haskell Pipes" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected].
