Hi all, I created some GADT code (see https://pastebin.com/vkJ3RzNA and below). When compiling this I get these errors: $ stack build parser-0.1.0.0: build (exe) Preprocessing executable 'parser' for parser-0.1.0.0.. Building executable 'parser' for parser-0.1.0.0.. [2 of 2] Compiling Main ( src/Main.hs, .stack-work/dist/x86_64-linux-tinfo6/Cabal-2.2.0.1/build/parser/parser-tmp/Main.o ) /home/hilco/workspaces/haskell/parser/src/Main.hs:30:18: error: • Couldn't match type ‘a’ with ‘E’ ‘a’ is a rigid type variable bound by the type signature for: h :: forall a. H (T a) at src/Main.hs:29:1-12 Expected type: T a Actual type: T E • In the expression: E' (E "E") In the expression: (E' (E "E"), []) In an equation for ‘h’: h [] = (E' (E "E"), []) • Relevant bindings include h :: H (T a) (bound at src/Main.hs:30:1) | 30 | h [] = (E' (E "E"), []) | ^^^^^^^^^^ /home/hilco/workspaces/haskell/parser/src/Main.hs:34:16: error: • Couldn't match type ‘B’ with ‘A’ Expected type: P Char (T A) Actual type: P Char (T B) • In the expression: b In the second argument of ‘tt’, namely ‘[a, b]’ In the expression: tt h [a, b] | 34 | tt' = tt h [a, b] | ^ -- While building custom Setup.hs for package parser-0.1.0.0 using: /home/hilco/.stack/setup-exe-cache/x86_64-linux-tinfo6/Cabal-simple_mPHDZzAJ_2.2.0.1_ghc-8.4.3 --builddir=.stack-work/dist/x86_64-linux-tinfo6/Cabal-2.2.0.1 build exe:parser --ghc-options " -ddump-hi -ddump-to-file -fdiagnostics-color=always" Process exited with code: ExitFailure 1 Why is "T a" not a more generic version of "T E"? Why doesn't "T E + T A + TB" yield "T a" as a generic type? Cheers, Hilco 1 {-# LANGUAGE GADTs #-} 2 3 module Main where 4 5 data PR t a = PR (Maybe ([t], a)) 6 data P t a = P ([t] -> PR t a) 7 8 a :: P Char (T A) 9 a = undefined 10 11 b :: P Char (T B) 12 b = undefined 13 14 data A = A 15 data B = B 16 data E = E String 17 18 data T a where 19 A' :: T A 20 B' :: T B 21 E' :: E -> T E 22 23 type H t = [Char] -> (t, [Char]) 24 type TT t = [Char] -> [t] 25 26 tt :: H t -> [P Char t] -> TT t 27 tt = undefined 28 29 h :: H (T a) 30 h [] = (E' (E "E"), []) 31 h (head:rest) = (E' (E ("E")), rest) 32 33 tt' :: [Char] -> [T a] 34 tt' = tt h [a, b] 35 36 main :: IO () 37 main = putStrLn "Hello"