Re-implement assert in terms of and_then

1 files changed, 5 insertions, 5 deletions

diff --git a/src/parcom/parser.cr b/src/parcom/parser.cr
index 2b058d4..d8f44c8 100644
--- a/src/parcom/parser.cr
+++ b/src/parcom/parser.cr
@@ -178,12 +178,12 @@ module Parcom
     # Creates a new parser from `self` that behaves the same, but fails
     # if the result does not satisfy the given predicate.
     def assert(f : U -> Bool) : Parser(T, U)
-      Parser(T, U).new("#{@name} (assertion)") do |tokens|
-        result = self.parse(tokens)
-        unless f.call(result.value)
-          raise ParserFail.new("Assertion failed for value #{result.value}")
+      self.and_then do |x|
+        if f.call(x)
+          Parser(T, U).pure(x)
+        else
+          Parser(T, U).flunk("Value <#{x}> failed assertion")
         end
-        result
       end
     end