typeguards.recapitulation.ts

// Parser is a function which takes unknown and returns T or null 
type Parser<T> = (val: unknown) => T | null;

// createTypeGuard is a function which takes a parser and returns a new function
// The returned function is a safe type guard for T
const createTypeGuard = <T>(parse: Parser<T>) => (value: unknown): value is T => {
    
  // By assuming that parser returns only T or null
  // We can say that value from parser is T when it is not a null
  return parse(value) !== null;
        
}