[Haskell-cafe] working my way through Data.Type.Equality...my head hurts...

Nicholls, Mark nicholls.mark at vimn.com
Tue Jan 13 13:21:49 UTC 2015


the 1st link is a bit challenging...the 2nd I can probably cope with for the moment, though I may be asking questions.

If you have any beginner level tutorial, then that’s even better..I can just about cope with associated types, type families and GADT...some of the other extensions I suspect disbelief for.

> What does “~” mean…(it’s something that comes out in error messages 
> when my types are all messed up)….
It means GHC knows that two types are equal.

> and then there’s a “=>” going on…in the middle of a signature….I know 
> “=>” in the context of “Num a => a -> a -> a”
Yeah, that looks scary. You use gcastWith when you need a value of type a but you have a value of type b with a proof that a and b are equal (here a proof is a value of a :~: b). I have a practical example, though perhaps it's not the simplest one:


This code is thought as an intermediate level tutorial - you might want to give it a try. Another example I have is here (I use the name subst, but it's identical to gcastWith):





This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited.

While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data.

Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us.

MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe.  MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc.  Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

More information about the Haskell-Cafe mailing list