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mapMaybe :: (a -> Maybe b) -> [a] -> [b]classy-prelude ClassyPrelude The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it is Just b, then b is included in the result list.
Examples
Using mapMaybe f x is a shortcut for catMaybes $ map f x in most cases:>>> import GHC.Internal.Text.Read ( readMaybe ) >>> let readMaybeInt = readMaybe :: String -> Maybe Int >>> mapMaybe readMaybeInt ["1", "Foo", "3"] [1,3] >>> catMaybes $ map readMaybeInt ["1", "Foo", "3"] [1,3]
If we map the Just constructor, the entire list should be returned:>>> mapMaybe Just [1,2,3] [1,2,3]
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esqueleto Database.Esqueleto No documentation available.
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esqueleto Database.Esqueleto The Maybe keyword goes after the type. This indicates that the column is nullable, and the generated Haskell code will have a Maybe type for it. Example:
User name Text Maybe
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esqueleto Database.Esqueleto As fromBaseId, but works on Maybe keys.
isFieldMaybe :: FieldDef -> Boolesqueleto Database.Esqueleto Check if the field is `Maybe a`
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esqueleto Database.Esqueleto Execute a subquery SELECT in a SqlExpr. This function is a shorthand for the common joinV . subSelect idiom, where you are calling subSelect on an expression that would be Maybe already. As an example, you would use this function when calling sum_ or max_, which have Maybe in the result type (for a 0 row query).
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esqueleto Database.Esqueleto -
esqueleto Database.Esqueleto.Experimental No documentation available.
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esqueleto Database.Esqueleto.Experimental The Maybe keyword goes after the type. This indicates that the column is nullable, and the generated Haskell code will have a Maybe type for it. Example:
User name Text Maybe
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esqueleto Database.Esqueleto.Experimental No documentation available.