diff options
Diffstat (limited to 'src/algebra')
| -rw-r--r-- | src/algebra/any.spad.pamphlet | 41 |
1 files changed, 19 insertions, 22 deletions
diff --git a/src/algebra/any.spad.pamphlet b/src/algebra/any.spad.pamphlet index 7bc2d368..b877b958 100644 --- a/src/algebra/any.spad.pamphlet +++ b/src/algebra/any.spad.pamphlet @@ -41,45 +41,42 @@ None():SetCategory == add \section{The Maybe domain} <<domain MAYBE Maybe>>= -import UnionType import CoercibleTo OutputForm import Boolean )abbrev domain MAYBE Maybe ++ Author: Gabriel Dos Reis ++ Date Created: August 20, 2008 -++ Also See: Union(T,"failed") +++ Date Last Modified: January 06, 2009 ++ Description: -++ This domain implements the notion of optional vallue, where +++ This domain implements the notion of optional value, where ++ a computation may fail to produce expected value. -++ Note: Ideally, this domain definition should be a one-liner. -++ That is currently impossible because of mismatch between -++ the `old representation' and `new representation' for domains. Maybe(T: CoercibleTo OutputForm): Public == Private where - Public == Join(UnionType,CoercibleTo OutputForm) with + Public == Join(UnionType,RetractableTo T, CoercibleTo OutputForm) with + just: T -> % + ++ maybe(x) injects the value `x' into %. _case: (%,[| T |]) -> Boolean ++ x case T returns true if x is actually a data of type T. _case: (%,[| nothing |]) -> Boolean ++ x case nothing evaluates true if the value for x is missing. - coerce: T -> % - ++ x::T injects the value x into %. - coerce: % -> T - ++ x::T tries to extract the value of T from the computation x. - ++ Produces a runtime error when the computation fails. autoCoerce: % -> T - ++ same as above but implicitly called by the compiler. + ++ autoCoerce is a courtesy coercion function used by the compiler + ++ in case it knows that `x' really is a T. nothing: % ++ represents failure. Private == add - Rep == Union(T,"nothing") - nothing == per("nothing"::Rep) - coerce(x: T): % == per(x::Rep) - coerce(x: %): T == rep(x)::T - autoCoerce x == rep(x)::T - x case T == rep x case T - x case nothing == rep x case "nothing" + nothing == %nothing$Lisp + just x == x : % + x case nothing == EQ(x,%nothing$Lisp)$Lisp + x case T == not(x case nothing) + autoCoerce x == x : T + coerce(x: T): % == just x + retractIfCan x == + x case T => x@T + "failed" coerce(x: %): OutputForm == - x case T => x::T::OutputForm - paren(empty()$OutputForm)$OutputForm + x case nothing => paren(empty()$OutputForm)$OutputForm + (x@T)::OutputForm + @ |