## Mercurial > urweb

### view lib/ur/option.ur @ 2086:3d22f761a4b7

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In checking signature subsumption, be sure to try constraints last.

author | Adam Chlipala <adam@chlipala.net> |
---|---|

date | Thu, 04 Dec 2014 20:22:39 -0500 |

parents | 36428d853c97 |

children |

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datatype t = datatype Basis.option val monad = mkMonad {Return = @@Some, Bind = fn [a] [b] (m1 : t a) (m2 : a -> t b) => case m1 of None => None | Some v => m2 v} fun eq [a] (_ : eq a) = mkEq (fn x y => case (x, y) of (None, None) => True | (Some x, Some y) => x = y | _ => False) fun ord [a] (_ : ord a) = mkOrd {Lt = fn x y => case (x, y) of (None, Some _) => True | (Some x, Some y) => x < y | _ => False, Le = fn x y => case (x, y) of (None, _) => True | (Some x, Some y) => x <= y | _ => False} fun isNone [a] x = case x of None => True | Some _ => False fun isSome [a] x = case x of None => False | Some _ => True fun mp [a] [b] f x = case x of None => None | Some y => Some (f y) fun bind [a] [b] f x = case x of None => None | Some y => f y fun get [a] (x : a) (o : option a) = case o of None => x | Some v => v fun unsafeGet [a] (o : option a) = case o of None => error <xml>Option.unsafeGet: encountered None</xml> | Some v => v