Subject: Re: impl typechecking errors
From: Vassily Litvinov (vass@cs.washington.edu)
Date: Fri Apr 06 2001 - 19:19:53 PDT
This message is tough to decipher, and I don't have a general recipe apart
from complaining to me. You may want to set the Vortex option "tc_debug"
to true to get a more meaningful error message, but even that wouldn't
help in this case.
Turns out that this is a *real* ITC error - which I didn't realize until
after I had written an almost complete apology for my constraint solver
being imperfect and in need of fixing.
The reason is that triple subtypes pair. As a result, for triple[T1,T2,T3]
to be hashable (and send of hash(triple[..],int) to be legal, according to
the signature), it suffices that the corresponding pair[T1,T2] be hashable.
As a result, send of hash(triple[T1,T2,T3],int) is legal if T1 and T2 are
hashable, but not necessarily T3. The method implementation for this send
naturally expects T3 to be hashable as well. Hence the error.
For a similar reason it is legal to compare (using <=) a triple and a
quadruple (given appropriate type params), which (I guess) shouldn't be
the case.
The right solution, of course, is to make triple a *private* subtype of
pair. (And of course first figure out what "private subtyping" should
mean.) Meanwhile there needs to be some workaround.....
Vass
PS. This lets me insert a plug for Vortex browser features. Typing in:
Vortex> subtypes triple 3 hashable 1
gives all the ways for a triple to subtype a hashable. You still may have
to contemplate the output to figure out what's going on...
On Thu, 5 Apr 2001, Craig Chambers wrote:
> I got the following weird itc error:
>
> comparable.cecil:143: implementation type error
> Kind: could not find legal instantiation of the callee
> Sig: signature hash(hashable[`T], range:int):int
> Recvs: -triple[A,B,C], *-
> Callee: method hash(p@:triple[`T1 <= hashable[T1], `T2 <= hashable[T2], `T3 <=
> hashable[T3]], range:int):int
> Instantiated signature:
> signature hash(hashable[`T], range:int):int
> Instantiated receivers:
> -triple[`#T1-2,`#T2-2,`#T3-2], *-
> Available constraints:
> SUB `#T-1 <= hashable[`#T-1]
> SUB `#T-1 <= pair[`#T2-3,`#T3-3]
> SUB `#T1-2 <= hashable[`#T2-3]
> SUB `#T2-2 <= hashable[`#T3-3]
> SUB `#T1-2 <= `#T2-3
> SUB `#T2-3 <= hashable[`#T2-3]
> SUB `#T2-2 <= `#T3-3
> SUB `#T3-3 <= hashable[`#T3-3]
> Callee instantiation constraints - could not solve them:
> hash(p@:triple[`T1 <= hashable[T1], `T2 <= hashable[T2], `T3 <= hashable[T3]],
> range:int):int
> hash(p@:triple[`T1 <= hashable[T1], `T2 <= hashable[T2], `T3 <= hashable[T3]],
> range:int):int
> hash(p@:triple[`T1 <= hashable[T1], `T2 <= hashable[T2], `T3 <= hashable[T3]],
> range:int):int
> SUB triple[`#T1-2,`#T2-2,`#T3-2] <= triple[`#T1-2,`#T2-2,`#T3-2]
> SUB int <= int
> SUB int <= int
>
> Vass, can you explain what the problem is with sending hash to a triple of
> hashables?
>
> -- Craig
>
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This archive was generated by hypermail 2b25 : Fri Apr 06 2001 - 19:21:04 PDT