Changes between Version 19 and Version 20 of Proposals/transformers

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Timestamp:
11/05/10 10:36:29 (4 years ago)
Author:
ross (IP: 81.98.180.212)
Comment:

refine Functor suggestion

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  • Proposals/transformers

    v19 v20  
    106106 This usually means that the instance you're defining is now defined in the {{{transformers}}} package.  You may wish to check it's the same, so you can just delete it. 
    107107 
    108 ''Could not deduce (Functor m) from the context (Monad m)'' 
    109  This will be in a situation where you're using a monad constructed with a transformer as a Functor.  If this is triggered by a use of {{{fmap}}}, you can replace it with {{{liftM}}} and the code will build with old and new versions of mtl.  Alternatively, you can replace the {{{Monad m}}} with {{{Functor m}}} to make it work with mtl-2, or add a {{{Functor m}}} constraint to make it work with old and new versions of mtl. 
     108''Could not deduce (Functor m) from the context (Monad m)[[BR]]arising from a use of fmap'' 
     109 This will be a situation where you're using a monad constructed with a transformer as a Functor.  You can replace {{{fmap}}} with {{{liftM}}} and the code will build with old and new versions of mtl. 
     110 
     111''Could not deduce (Functor m) from the context (Monad m)[[BR]]arising from the superclasses of an instance declaration'' 
     112 This will be another situation where you're using a monad constructed with a transformer as a Functor.  You can replace the {{{Monad m}}} with {{{Functor m}}} to make it work with mtl-2, or add a {{{Functor m}}} constraint to make it work with old and new versions of mtl. 
    110113 
    111114''Not in scope: data constructor State''