Opaleye.Internal.MaybeFields

The Opaleye analogue of Maybe. A value of type MaybeFields a either contains a value of type a, or it contains nothing.

The Opaleye analogue of Nothing.

The Opaleye analogue of const Nothing. Can be useful to avoid type inference problems, because it doesn't pick up a type class constraint.

The Opaleye analogue of Just. Equivalent to pure.

The Opaleye analogue of maybe

Use a Haskell \case expression to pattern match on a MaybeFields.

example :: MaybeFields (Field SqlInt4) -> Field SqlInt4
example mf = matchMaybe mf $ \case
  Nothing -> 0
  Just x  -> x * 100

The Opaleye analogue of fromMaybe

The Opaleye analogue of maybeToList. Unless you are using arrow notation you'll probably find catMaybeFields easier to use.

The Opaleye analogue of catMaybes. Most commonly you will want to use this at type

catMaybeFields :: Select (MaybeFields a) -> Select a

An example to demonstrate how the functionality of (lateral) LEFT JOIN can be recovered using optional.

An example to demonstrate how the functionality of optionalRestrict can be recovered using optional.

Convert NULL to nothingFields and non-NULL to a justFields

Convert nothingFields to NULL to a justFields to non-NULL

This is not safe in general because it relies on p not doing anything observable with the as if mfPresent is false. In particular, it won't work for Distinctspec because it does indeed look at the mfFields to check distinctness.

This is only safe if d is OK with having nulls passed through it when they claim to be non-null.

This is only safe if b is OK with having nulls passed through it when they claim to be non-null.

This is only safe if col is OK with having nulls passed through it when they claim to be non-null.