Greg Best wrote:
---------------------------------NMEATest.hs----------------------------------
module NMEATest where
data ZeroOrMore a = NoVal | SingleVal a | MultiVal [a] deriving (Eq,Ord,Show) type Sentence = String newtype NMEAParser a = NMEAParser(Sentence -> (ZeroOrMore a, Sentence))
instance Monad NMEAParser where return a = NMEAParser(\s -> (SingleVal a,s)) NMEAParser k >>= f = NMEAParser(\s0 -> let (r1, s1) = k s0 k2 = f r1 (r2, s2) = k2 s1 in (r1,s2)) ------------------------------------------------------------------------------------
NMEATest.hs:26:45: Occurs check: cannot construct the infinite type: a = ZeroOrMore a
f and r1 have types f :: a -> NMEAParser b r1 :: ZeroOrMore a so that your use of f r1 forces a = ZeroOrMore a which cannot be the case. You have to deconstruct r1 and do something appropriate for the three different cases.
NMEATest.hs:28:42: Couldn't match expected type `b' against inferred type `a' Expected type: ZeroOrMore b Inferred type: ZeroOrMore a
Your >>= returns r1, the result of executing the left hand side action, but the overall result of executing (k >>= f) should be the result of the right hand side, i.e. r2. Note that r1 and r2 have indeed the types r1 :: ZeroOrMore a r2 :: ZeroOrMore b. Tillmann