Functional pattern matching for typescript/javascript

Define complex patterns as functions.

Portable reusable logic.

Strictly typed pattern matching using typescript

consttestSubject=222;

constresult=fcase(testSubject)(match=>{match.case('222').then(shouldNotCall('string("222") !== number(222)')).case(22).then(shouldNotCall('22 !== testSubject')).case(22).then(shouldNotCall('22 !== testSubject')).case(222).then(()=>{testSuccess('Yay!! 222 === testSubject');return3;}).case(222).then(shouldNotCall('conditions already satisfied')).default(shouldNotCall('Conditions already satisfied'));});

result===3;// true - since matching block returned 3

Unlike traditional switch-case, fcase always break after a match. To keep continuing, an explicit call to matchNext must be made.

Modifier and didMatch

Modifier is an object consisting of 2 control flow modifier functions {matchNext,__DO__NEXT__} passed as 1st parameter to then callback. didMatch is the 2nd parameter to then callback. It is a boolean value resolved from equality checking testSubject to case value. This will be false when the previous block called __DO__NEXT__ and condition does not match.

__DO__NEXT__ is named so, to signify the oddness of using it and as a reminder that it is not recommended for regular usage. It is added as an explicit call, only to feature match native switch-case statement.

These can be accessed as follows:

constresult=fcase(testSubject)(match=>{match
...
.case('first matching condition in this chain').then(({matchNext,__DO__NEXT__},didMatch)=>{// didMatch is true// if matchNext/__DO__NEXT__ is not called, execution will break here// matchNext executes the next block that satisfy case condition.matchNext();return2;}).case('someCondition that will not matches').then(shouldNotExecute("condition doesn't match")).case('someCondition that matches').then(({matchNext,__DO__NEXT__},didMatch)=>{// didMatch is true// if matchNext/__DO__NEXT__ is not called, execution will break here// __DO__NEXT__ executes block right after the calling block__DO__NEXT__();return3;}).case('someCondition-that-will-never-match').then(({matchNext,__DO__NEXT__},didMatch)=>{// `didMatch` is false// this block executes because of __DO__NEXT__ in previous block eventhough case condition doesn't match testSubject// next block will not executereturn4;})...

In the above example return value will be the value returned bu last executing block, despite what caused it to execute

result===4// `true` since last executing block returned 4