Changes between Version 7 and Version 8 of OverloadedLists

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01/30/13 04:01:11 (4 months ago)
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  • OverloadedLists

    v7 v8  
    5757}}} 
    5858 
    59 GHC, during the typechecking and desugaring phases, uses whatever is in scope 
    60 with the names of {{{fromList}}} and {{{fromListN}}} (i.e., {{{fromList}}} and 
    61 {{{fromListN}}} are rebindable). 
    62  
    63 That said, the {{{GHC.Exts}}} module exports the {{{FromList}}} class that can 
    64 be used to overload {{{fromListN}}} and {{{fromListN}}} for different 
    65 structures. The type class is defined as follows: 
    66  
    67 {{{ 
    68 class FromList l where 
    69   type Item l 
    70   fromList  :: [Item l] -> l 
    71   fromListN :: Int -> [Item l] -> l 
    72   fromListN _ = fromList 
    73 }}} 
    74  
    75 The {{{FromList}}} class and its methods are intended to be used in 
    76 conjunction with the {{{OverloadedLists}}} extension. The {{{Item}}} type 
    77 function returns the type of items of the structure {{{l}}}. The fromList 
    78 function constructs the structure {{{l}}} from the given list of {{{Item l}}}. 
    79 The {{{fromListN}}} function takes the input list's length as a hint. Its 
    80 behaviour should be equivalent to {{{fromList}}}. The hint can be used for 
    81 more efficient construction of the structure {{{l}}} compared to 
    82 {{{fromList}}}. If the given hint is not equal to the input list's length the 
    83 behaviour of {{{fromListN}}} is not specified. 
    84  
    85 In the following, we give several example instances of the {{{FromList}}} type 
    86 class: 
    87  
    88 {{{ 
    89 instance FromList [a] where 
    90   type Item [a] = a 
    91   fromList = id 
    92  
    93 instance (Ord a) => FromList (Set a) where 
    94   type Item (Set a) = a 
    95   fromList = Set.fromList 
    96  
    97 instance (Ord k) => FromList (Map k v) where 
    98   type Item (Map k v) = (k,v) 
    99   fromList = Map.fromList 
    100  
    101 instance FromList (IntMap v) where 
    102   type Item (IntMap v) = (Int,v) 
    103   fromList = IntMap.fromList 
    104  
    105 instance FromList Text where 
    106   type Item Text = Char 
    107   fromList = Text.pack 
    108  
    109 instance FromList (Vector a) where 
    110   type Item (Vector a) = a 
    111   fromList  = Vector.fromList 
    112   fromListN = Vector.fromListN 
    113 }}} 
    114  
    115 == TODO: Overloading Pattern Matching on Lists == 
    116  
    117 One way to overload list patterns is to replace the {{{FromList}}} class from 
    118 the previous section with the {{{IsList}}} class defined as follows: 
    119  
    120 {{{ 
    121 class IsList l where 
    122   type Item l 
    123   fromList  :: [Item l] -> l 
    124   toList    :: l -> [Item l] 
    125  
    126   fromListN :: Int -> [Item l] -> l 
    127   fromListN _ = fromList 
    128 }}} 
    129  
    130 This class allows us to view the structure {{{l}}} as a list. Now, the 
    131 {{{toList}}} can be used to overload the list notation in patters. 
    132 For example, When the {{{OverloadedLists}}} extension is turned on, the 
     59List patterns are also overloaded. When the {{{OverloadedLists}}} extension is turned on, the 
    13360definitions 
    13461 
    … …  
    14572}}} 
    14673 
    147 Here, just like for expressions, we propose to overload list patterns that use 
    148 square brackets. The {{{:}}} infix operator will remain list specific both in 
    149 expressions and patterns. In other words, {{{:}}} is not overloaded. 
     74GHC, during the typechecking and desugaring phases, uses whatever is in scope 
     75with the names of {{{fromList}}} and {{{fromListN}}} (i.e., {{{fromList}}} and 
     76{{{fromListN}}} are rebindable). 
     77 
     78That said, the {{{GHC.Exts}}} module exports the {{{IsList}}} class that can 
     79be used to overload {{{fromListN}}} and {{{fromListN}}} for different 
     80structures. The type class is defined as follows: 
     81 
     82{{{ 
     83class IsList l where 
     84  type Item l 
     85  fromList  :: [Item l] -> l 
     86  toList    :: l -> [Item l] 
     87 
     88  fromListN :: Int -> [Item l] -> l 
     89  fromListN _ = fromList   
     90}}} 
     91 
     92The {{{IsList}}} class and its methods are intended to be used in 
     93conjunction with the {{{OverloadedLists}}} extension. The {{{Item}}} type 
     94function returns the type of items of the structure {{{l}}}. The fromList 
     95function constructs the structure {{{l}}} from the given list of {{{Item l}}}. 
     96The {{{fromListN}}} function takes the input list's length as a hint. Its 
     97behaviour should be equivalent to {{{fromList}}}. The hint can be used for 
     98more efficient construction of the structure {{{l}}} compared to 
     99{{{fromList}}}. If the given hint is not equal to the input list's length the 
     100behaviour of {{{fromListN}}} is not specified. 
    150101 
    151102The instances of the {{{IsList}}} class should satisfy the following 
    … …  
    156107}}} 
    157108 
    158 The example {{{FromList}}} instances from the previous section can be extended 
    159 into the {{{IsList}}} instances as follows: 
     109In the following, we give several example instances of the {{{IsList}}} type 
     110class: 
    160111 
    161112{{{ 
    … …  
    163114  type Item [a] = a 
    164115  fromList = id 
    165   toList = id 
     116  toList   = id 
    166117 
    167118instance (Ord a) => IsList (Set a) where 
    168119  type Item (Set a) = a 
    169120  fromList = Set.fromList 
    170   toList = Set.toList 
     121  toList   = Set.toList 
    171122 
    172123instance (Ord k) => IsList (Map k v) where 
    173124  type Item (Map k v) = (k,v) 
    174125  fromList = Map.fromList 
    175   toList = Map.toList 
     126  toList   = Map.toList 
    176127 
    177128instance IsList (IntMap v) where 
    178129  type Item (IntMap v) = (Int,v) 
    179130  fromList = IntMap.fromList 
    180   toList = IntMap.toList 
     131  toList   = IntMap.toList 
    181132 
    182133instance IsList Text where 
    183134  type Item Text = Char 
    184135  fromList = Text.pack 
    185   toList = Text.unpack 
     136  toList   = Text.unpack 
    186137 
    187138instance IsList (Vector a) where 
    188139  type Item (Vector a) = a 
    189140  fromList  = Vector.fromList 
     141  toList    = Vector.toList 
    190142  fromListN = Vector.fromListN 
    191   toList = Vector.toList 
     143 
    192144}}} 
    193  
    194 An alternative name for the {{{IsList}}} class could be {{{ListRep}}} (meaning 
    195 representable as list). This is to emphasise that its instances should be 
    196 representable as lists, it is not required for them to be lists. 
    197145 
    198146== Further GHC improvements/extensions that may benefit {{{OverloadedLists}}} ==