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Within LTS Haskell 24.38 (ghc-9.10.3)
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classy-prelude ClassyPrelude No documentation available.
fromMaybe :: a -> Maybe a -> aclassy-prelude ClassyPrelude The fromMaybe function takes a default value and a Maybe value. If the Maybe is Nothing, it returns the default value; otherwise, it returns the value contained in the Maybe.
Examples
Basic usage:>>> fromMaybe "" (Just "Hello, World!") "Hello, World!"
>>> fromMaybe "" Nothing ""
Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:>>> import GHC.Internal.Text.Read ( readMaybe ) >>> fromMaybe 0 (readMaybe "5") 5 >>> fromMaybe 0 (readMaybe "") 0
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classy-prelude ClassyPrelude The listToMaybe function returns Nothing on an empty list or Just a where a is the first element of the list.
Examples
Basic usage:>>> listToMaybe [] Nothing
>>> listToMaybe [9] Just 9
>>> listToMaybe [1,2,3] Just 1
Composing maybeToList with listToMaybe should be the identity on singleton/empty lists:>>> maybeToList $ listToMaybe [5] [5] >>> maybeToList $ listToMaybe [] []
But not on lists with more than one element:>>> maybeToList $ listToMaybe [1,2,3] [1]
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