|  If we indeed had something like
| 
|      head :: Partial => [a] -> a
| 
|  that would be both informative and fairly straightforward to explain to
|  students, for example. (Even if it is not clear to me that a type class
|  really is the right way to express partiality of functions: I always thought
|  information about partiality ought to be tied to the function arrow.)

OK -- that sounds promising.  It's what Richard suggested earlier, and sounds pretty good to me.

Simon


|  -----Original Message-----
|  From: Henrik.Nilsson@nottingham.ac.uk <Henrik.Nilsson@nottingham.ac.uk>
|  Sent: 09 June 2021 14:45
|  To: Simon Peyton Jones <simonpj@microsoft.com>; Henrik Nilsson
|  <Henrik.Nilsson@nottingham.ac.uk>; libraries@haskell.org
|  Subject: Re: RFC: Add HasCallStack constraint to partial Data.List
|  functions.
| 
|   > I'm not sure I really agree with that.  There is a rich literature on  >
|  effect systems, which decorate types with information about what  > effects
|  the function has: exceptions, divergence, IO, and the like.
|    > So type like
|   >   head :: Partial => [a] -> a
|   > where 'Partial =>' expresses the fact that calling this function  > might
|  lead to a call of 'error' doesn't seem inherently something  > that doesn't
|  belong in a type system.
| 
|  I, of course, agree that partiality is an effect. And I have no issues with
|  effects being reflected in the type system.
|  We do that all the time with e.g. monads.
| 
|  If we indeed had something like
| 
|      head :: Partial => [a] -> a
| 
|  that would be both informative and fairly straightforward to explain to
|  students, for example. (Even if it is not clear to me that a type class
|  really is the right way to express partiality of functions: I always thought
|  information about partiality ought to be tied to the function arrow.)
| 
|  My point is that "HasCallStack" strongly suggest a specific approach to
|  monitor the behaviour of a function in case it goes wrong.
| 
|  To me, at least, that is very operational.
| 
|  And I would struggle to explain
| 
|      head :: HasCallStack => [a] -> a
| 
|  beyond saying "it's just something that sometimes will help you with
|  debugging", and deeply hoping no clever student would ask about the lack of
|  similar annotations for other partial functions.
| 
|  Best,
| 
|  /Henrik
| 
| 
| 
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| 
| 
| 

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