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Add Homogeneous and HomogeneousRowList for homogeneous rows #20

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merged 8 commits into from
Dec 10, 2017
Merged

Add Homogeneous and HomogeneousRowList for homogeneous rows #20

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8b39b00
Add RecordOf and FieldOf for homogeneous records
paulyoung Aug 10, 2017
17d4228
Rename to `Homogenous` and move to `Row` type
paulyoung Aug 24, 2017
0d25ad4
Add comment explaining Homogeneous class
paulyoung Sep 12, 2017
3d51ac4
Add missin fun dep
paulyoung Sep 21, 2017
6d179f8
Add Type.Row.Homogeneous
paulyoung Nov 20, 2017
20fdbe6
Remove unwanted comment
paulyoung Dec 6, 2017
fe087b6
Rename `FieldOf` to `HomogenousRowList`
paulyoung Dec 6, 2017
632538d
Rename instances to coincide previous renaming
paulyoung Dec 6, 2017
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18 changes: 18 additions & 0 deletions src/Type/Row.purs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,10 @@ module Type.Row
, class ListToRow
) where


import Type.Equality (class TypeEquals)


data RProxy (row :: # Type) = RProxy

-- Must not be exported
Expand Down Expand Up @@ -70,3 +74,17 @@ instance listToRowCons
:: ( ListToRow tail tailRow
, RowCons label ty tailRow row )
=> ListToRow (Cons label ty tail) row

-- | Ensure that every field in a row has the same type.
class Homogeneous (row :: # Type) fieldType
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Also missing a functional dependency here.

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Good catch.

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Am I correct in thinking that the right thing to do here is | row -> fieldType?

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Could you please add a comment?

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Perhaps a value-level function fieldType :: RProxy row -> Proxy fieldType would be handy as well.

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Will do.

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I though I'd asked this already but can't see it now.

Is | row -> fieldType missing here?

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Yes I think so

instance homogeneous
:: ( RowToList row fields
, FieldOf fields fieldType )
=> Homogeneous row fieldType

class FieldOf (rowList :: RowList) fieldType | rowList -> fieldType
instance fieldOfCons
:: ( FieldOf tail fieldType
, TypeEquals fieldType fieldType2 )
=> FieldOf (Cons symbol fieldType tail) fieldType2
instance fieldOfNil :: FieldOf Nil fieldType