1 files changed, 45 insertions, 0 deletions
diff --git a/src/__OLD_parcom/assert.cr b/src/__OLD_parcom/assert.cr
new file mode 100644
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--- /dev/null
+++ b/src/__OLD_parcom/assert.cr
@@ -0,0 +1,45 @@
+require "./parser.cr"
+
+module Parcom
+  # `Assert` is a `Parser` that runs another `Parser` and then
+  # performs a test on its result. The parser will then either fail if
+  # the result does not pass the test, or forward it on if it does.
+  #
+  # Example:
+  # ```
+  # letter = Assert.new(AnyToken(Char).new) { |x| x.letter? }
+  # non_letter = Assert.new(AnyToken(Char).new) { |x| !x.letter? }
+  # input = Tokens.from_string("hi")
+  # letter.parse(input) # this succeeds
+  # non_letter.parse(input) # this fails
+  # ```
+  #
+  # `Assert` instance can also be created by calling
+  # `Parser#assert` on any parser:
+  # ```
+  # # This parser is identical to the above example.
+  # letter = AnyToken.new(Char).assert { |x| x.letter? }
+  # ```
+  class Assert(T, V) < Parser(T, V)
+    # Accepts the parser to run and a `Bool`-retuning block
+    # containing the test to perform on the other parser's result.
+    def initialize(@p : Parser(T, V), &block : V -> Bool)
+      @f = block
+    end 
+
+    # Runs the parser it was initialized with, and returns that parser's
+    # result if it passes the provided test. Raises `ParserFail` otherwise.
+    def parse(tokens : Tokens(T)) : Result(T, V)
+      result = @p.parse(tokens)
+    rescue ex : ParserFail
+      raise ParserFail.new("Assert (pre-assertion): #{ex.message}")
+    else
+      unless @f.call(result.value)
+        raise ParserFail.new("Assert: predicate failed for <#{result.value}>")
+      end
+
+      return result
+    end
+  end
+end
+