Difference between revisions of "DDC/FieldProjections"
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instance Out Vector where |
instance Out Vector where |
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− | + | out vec = println $ parens (show vec.x % "," % show vec.y) |
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main () |
main () |
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Line 24: | Line 24: | ||
<haskell> |
<haskell> |
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project Vector where |
project Vector where |
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− | magnitude (Vector x y) |
+ | magnitude (Vector x y) = sqrt (x * x + y * y) |
− | = sqrt (x * x + y * y) |
||
</haskell> |
</haskell> |
Revision as of 11:27, 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'
Custom projections
We can also define our own projections.
project Vector where
magnitude (Vector x y) = sqrt (x * x + y * y)