Changes between Version 4 and Version 5 of UndecidableInstances


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Timestamp:
Feb 19, 2006 1:14:14 AM (8 years ago)
Author:
ross@…
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  • UndecidableInstances

    v4 v5  
    22= Allow Undecidable Instances = 
    33 
    4 '''[search:?q="UndecidableInstances"&ticket=on Ticket(s)]''' 
    5  
    64== Brief Explanation == 
    75 
    8 There are no restrictions on the form of instance declarations (see FlexibleInstances). As a result, context reduction need not terminate, so implementations impose some limit on the depth of inference. 
     6There are no restrictions on the form of instance declarations (see FlexibleInstances). 
     7Implementations ensure termination by making context reduction fail if it exceeds some depth limit. 
    98 
    109== References == 
    1110 * [http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#undecidable-instances Undecidable instances] in the GHC User's guide. 
    1211 
     12== Tickets == 
     13[[TicketQuery(description~=UndecidableInstances)]] 
     14 
    1315== Pros == 
    14  * offered by GHC and Hugs with appropriate options 
    15  * flexible 
     16 * allows safe instances that would be rejected by conservative termination checks (see FlexibleInstances). 
     17 * offered by GHC and Hugs with appropriate options for several years. 
     18 * experience has shown that increasing the depth limit rarely makes a difference in which programs are accepted. 
    1619 
    1720== Cons == 
    18  * no clear boundary 
     21 * the boundary between legal and illegal programs is unclear, and possibly implementation-dependent.