DDC/FieldProjections: Difference between revisions
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== Data | == Data fields == | ||
When data types are defined using field names, we can use the projection operator <hask>(.)</hask> to select the fields. | When data types are defined using field names, we can use the projection operator <hask>(.)</hask> to select the fields. | ||
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instance Out Vector where | instance Out Vector where | ||
out vec = println $ parens (show x % "," % show y) | out vec = println $ parens (show vec.x % "," % show vec.y) | ||
main () | main () | ||
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out vec.x -- prints '3.0' | out vec.x -- prints '3.0' | ||
out vec.y -- prints '4.0' | out vec.y -- prints '4.0' | ||
</haskell> | |||
== Custom projections == | |||
Revision as of 11:24, 18 March 2008
Data fields
When data types are defined using field names, we can use the projection operator (.) to select the fields.
data Vector
= Vector { x :: Float; y :: Float; }
instance Out Vector where
out vec = println $ parens (show vec.x % "," % show vec.y)
main ()
= do vec = Vector 3.0 4.0
out vec -- prints '(3.0, 4.0)'
out vec.x -- prints '3.0'
out vec.y -- prints '4.0'