22 Jan
2013
22 Jan
'13
7:12 p.m.
On Tue, 22 Jan 2013, John Wiegley wrote:
Use 'onlyIf' with AndM and AndMT to guard later statements, which are only evaluated if every preceding 'onlyIf' evaluates to True. For example:
foo :: AndM Int foo = do onlyIf (True == True) return 100 onlyIf (True == True) return 150 onlyIf (True == False) return 200
When run with `evalAndM foo (-1)` (where (-1) provides a default value), 'foo' returns 150.
Does the And monad fulfill the monad laws? In a proper monad an interim (return x) (without a '<-') is a no-op.