Re: [Haskell-cafe] Problems with type families

6 Jul 2015

      Hi Silvio, thanks for the answer.
...
What's the error message? Is a bit hard without that.
The error message is: (the source code is attached)

src/DSP/Sig.hs:41:25:
     Could not deduce (TFunOut t1 e ~ TFunOut t1 e0)
     from the context (SignalT t1 e,
                       SignalT t2 e,
                       TFunOut t2 e ~ TFunIn t1 e)
       bound by the instance declaration at src/DSP/Sig.hs:39:10-95
     NB: ‘TFunOut’ is a type function, and may not be injective
     The type variable ‘e0’ is ambiguous
     Expected type: TFunOut (ComposeT t1 t2) e
       Actual type: TFunOut t1 e0
     Relevant bindings include
       x :: TFunIn (ComposeT t1 t2) e (bound at src/DSP/Sig.hs:41:21)
       transform :: ComposeT t1 t2
                    -> TFunIn (ComposeT t1 t2) e -> TFunOut (ComposeT t1 
t2) e
         (bound at src/DSP/Sig.hs:41:9)
     In the expression:
       (transform (undefined :: t1) . transform (undefined :: t2)) x
     In an equation for ‘transform’:
         transform _ x
           = (transform (undefined :: t1) . transform (undefined :: t2)) x

src/DSP/Sig.hs:41:26:
     Could not deduce (TFunIn t1 e0 ~ TFunOut t2 e1)
     from the context (SignalT t1 e,
                       SignalT t2 e,
                       TFunOut t2 e ~ TFunIn t1 e)
       bound by the instance declaration at src/DSP/Sig.hs:39:10-95
     The type variables ‘e0’, ‘e1’ are ambiguous
     Expected type: TFunOut t2 e1 -> TFunOut t1 e0
       Actual type: TFunIn t1 e0 -> TFunOut t1 e0
     Relevant bindings include
       x :: TFunIn (ComposeT t1 t2) e (bound at src/DSP/Sig.hs:41:21)
       transform :: ComposeT t1 t2
                    -> TFunIn (ComposeT t1 t2) e -> TFunOut (ComposeT t1 
t2) e
         (bound at src/DSP/Sig.hs:41:9)
     In the first argument of ‘(.)’, namely
       ‘transform (undefined :: t1)’
     In the expression:
       transform (undefined :: t1) . transform (undefined :: t2)

src/DSP/Sig.hs:41:56:
     Could not deduce (TFunIn t2 e1 ~ TFunIn t2 e)
     from the context (SignalT t1 e,
                       SignalT t2 e,
                       TFunOut t2 e ~ TFunIn t1 e)
       bound by the instance declaration at src/DSP/Sig.hs:39:10-95
     NB: ‘TFunIn’ is a type function, and may not be injective
     The type variable ‘e1’ is ambiguous
     Expected type: TFunIn t2 e -> TFunOut t2 e1
       Actual type: TFunIn t2 e1 -> TFunOut t2 e1
     Relevant bindings include
       x :: TFunIn (ComposeT t1 t2) e (bound at src/DSP/Sig.hs:41:21)
       transform :: ComposeT t1 t2
                    -> TFunIn (ComposeT t1 t2) e -> TFunOut (ComposeT t1 
t2) e
         (bound at src/DSP/Sig.hs:41:9)
     In the second argument of ‘(.)’, namely
       ‘transform (undefined :: t2)’
     In the expression:
       transform (undefined :: t1) . transform (undefined :: t2)
Failed, modules loaded: none.
...
The problem is probably that you have to explicitly state everything.
Maybe
TFunIn (ComposeT t1 t2) e ~ TFunOut (SignalT t2 e)
The problem with typefamilies is that often things can not be used as
you would expect unless you state things explicitly.
It is because the instance declaration can not know that TFunIn is
always the same. You could import two different ones from different
modules and thus convert one type into another.
closed typefamilies can sometimes help too.
-- 
Leza Morais Lutonda, Lemol-C
Electronic and Telecommunication Engineer
Software Development and Architecture Enthusiast
http://lemol.github.io

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