Good evening,
the following says it all:
~% /var/tmp/ghc/bin/ghci /tmp/foo.hs
___ ___ _
/ _ \ /\ /\/ __(_)
/ /_\// /_/ / / | | GHC Interactive, version 6.4.20050215, for Haskell
98.
/ /_\\/ __ / /___| | http://www.haskell.org/ghc/
\____/\/ /_/\____/|_| Type :? for help.
Loading package base-1.0 ... linking ... done.
Compiling Main ( /tmp/foo.hs, interpreted )
/tmp/foo.hs:64:6:
No instance for (Show c)
arising from use of `show' at /tmp/foo.hs:64:6-9
Probable fix:*** Exception: typecheck/TcSimplify.lhs:2389:8-82:
Non-exhaustive patterns in function add_ors
>
The "work-in-progress" source causing it is attached.
Happy hacking,
Remi
--
Nobody can be exactly like me. Even I have trouble doing it.
{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}
-- static Peano constructors and numerals
data Zero = Zero deriving Show
data One = One deriving Show
infixl 9 :@
data (Number a, Digit b) => a :@ b = a :@ b deriving Show
class Digit a
instance Digit Zero
instance Digit One
class Number a
instance Number Zero
instance Number One
instance (Number a, Digit b) => Number (a :@ b)
--- Pretty printing of numbers ---
class PrettyNum a where
prettyNum :: a -> String
instance PrettyNum Zero where
prettyNum _ = "0"
instance PrettyNum One where
prettyNum _ = "1"
instance (Number a, Digit b, PrettyNum a, PrettyNum b)
=> PrettyNum (a :@ b) where
prettyNum ~(a:@b)
= prettyNum a ++ prettyNum b
--- Digit addition ---
class (Number a, Digit b, Number c)
=> AddDigit a b c | a b -> c where
addDigit :: a -> b -> c
addDigit = undefined
instance Number a => AddDigit a Zero a
instance AddDigit Zero One One
instance AddDigit One One (One:@Zero)
instance Number a => AddDigit (a:@Zero) One (a:@One)
instance AddDigit a One a'
=> AddDigit (a:@One) One (a':@Zero)
--- Addition ---
class (Number a, Number b, Number c)
=> Add a b c | a b -> c where
add :: a -> b -> c
add = undefined
instance Number n => Add n Zero n
instance Add Zero One One
instance Add One One (One:@One)
instance Number n
=> Add (n:@Zero) One (n:@One)
instance AddDigit n One r'
=> Add (n:@One) One (r':@Zero)
instance (Number n1, Digit d1, Number n2, Digit n2
,Add n1 n2 nr', AddDigit (d1:@nr') d2 r)
=> Add (n1:@d1) (n2:@d2) r
foo = show $ add (One:@Zero) (One:@One)
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