I have a function which, without a type annotation, types as: *Codec.ASN1.BER> :t choiceAux choiceAux :: forall (m :: * -> *) e e1. (MonadState [Maybe Encoding] m, MonadState [Maybe Encoding] (StateT [Maybe Encoding] m), MonadError e (StateT [Maybe Encoding] m), MonadError e1 m) => (TagPlicity, NamedType) -> Encoding -> m Defaulted But if I try to give it a type annotation of choiceAux :: (MonadState [Maybe Encoding] m, MonadState [Maybe Encoding] (StateT [Maybe Encoding] m), MonadError e (StateT [Maybe Encoding] m), MonadError e1 m) => (TagPlicity, NamedType) -> Encoding -> m Defaulted I get Codec/ASN1/BER.hs:66:0: Quantified type variable `e1' is unified with another quantified type variable e When trying to generalise the type inferred for `choiceAux' Signature type: forall (m :: * -> *) e e1. (MonadState [Maybe Encoding] m, MonadState [Maybe Encoding] (StateT [Maybe Encoding] m), MonadError e (StateT [Maybe Encoding] m), MonadError e1 m) => (TagPlicity, NamedType) -> Encoding -> m Defaulted Type to generalise: (TagPlicity, NamedType) -> Encoding -> m Defaulted In the type signature for `choiceAux' When generalising the type(s) for tc, choiceAux, k Changing e1 to e makes it typecheck. 1. Can anyone explain why the typechecker rejects the type it had inferred for itself? 2. I thought StateT guaranteed that the lifted monad was a state monad so why is the typechecker adding MonadState ... (StateT ... ) to the list of constraints? Ditto for MonadError ... (StateT ... )? Thanks, Dominic. PS What I thought I was going to get was choiceAux :: forall (m :: * -> *) e. (MonadError e m, MonadState [Maybe Encoding] m) => (TagPlicity, NamedType) -> Encoding -> m Defaulted and this is what I have now specified.
I had this exact same issue when I swapped e and e1 by mistake. Does your code work right without the type signature or does it just compile? On Jan 2, 2006, at 8:59 AM, Dominic Steinitz wrote:
Codec/ASN1/BER.hs:66:0: Quantified type variable `e1' is unified with another quantified type variable e When trying to generalise the type inferred for `choiceAux' Signature type: forall (m :: * -> *) e e1. (MonadState [Maybe Encoding] m, MonadState [Maybe Encoding] (StateT [Maybe Encoding] m), MonadError e (StateT [Maybe Encoding] m), MonadError e1 m) => (TagPlicity, NamedType) -> Encoding -> m Defaulted Type to generalise: (TagPlicity, NamedType) -> Encoding -> m Defaulted In the type signature for `choiceAux' When generalising the type(s) for tc, choiceAux, k
Changing e1 to e makes it typecheck.
On Monday 02 Jan 2006 9:52 am, Joel Reymont wrote:
I had this exact same issue when I swapped e and e1 by mistake.
Does your code work right without the type signature or does it just compile?
It seems to work ok. I am able to parse an attribute certificate generated by a different (java) implementation + the code passes various other tests. I was curious as to why the type inferencer was giving me a result it then wouldn't accept itself. Dominic.
Hello Dominic, Monday, January 02, 2006, 11:59:53 AM, you wrote: *Codec.ASN1.BER>> :t choiceAux DS> choiceAux :: forall (m :: * -> *) e e1. DS> (MonadState [Maybe Encoding] m, DS> MonadState [Maybe Encoding] (StateT [Maybe Encoding] m), DS> MonadError e (StateT [Maybe Encoding] m), DS> MonadError e1 m) => DS> (TagPlicity, NamedType) -> Encoding -> m Defaulted DS> But if I try to give it a type annotation of DS> choiceAux :: (MonadState [Maybe Encoding] m, DS> MonadState [Maybe Encoding] (StateT [Maybe Encoding] m), DS> MonadError e (StateT [Maybe Encoding] m), DS> MonadError e1 m) => DS> (TagPlicity, NamedType) -> Encoding -> m Defaulted you omited `forall` specifier, which, i think, makes the difference -- Best regards, Bulat mailto:bulatz@HotPOP.com
On Monday 02 Jan 2006 10:59 am, Bulat Ziganshin wrote:
Hello Dominic,
Monday, January 02, 2006, 11:59:53 AM, you wrote:
*Codec.ASN1.BER>> :t choiceAux DS> choiceAux :: forall (m :: * -> *) e e1. DS> (MonadState [Maybe Encoding] m, DS> MonadState [Maybe Encoding] (StateT [Maybe Encoding] m), DS> MonadError e (StateT [Maybe Encoding] m), DS> MonadError e1 m) => DS> (TagPlicity, NamedType) -> Encoding -> m Defaulted
DS> But if I try to give it a type annotation of
DS> choiceAux :: (MonadState [Maybe Encoding] m, DS> MonadState [Maybe Encoding] (StateT [Maybe Encoding] m), DS> MonadError e (StateT [Maybe Encoding] m), DS> MonadError e1 m) => DS> (TagPlicity, NamedType) -> Encoding -> m Defaulted
you omited `forall` specifier, which, i think, makes the difference Bulat,
I get the same error and furthermore I thought that any free type variables were implicitly quantified, e.g., foobar :: Eq a => a -> a -> Bool foobar x y = if x == y then True else False *Main> :t foobar foobar :: forall a. (Eq a) => a -> a -> Bool Dominic.
participants (3)
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Bulat Ziganshin -
Dominic Steinitz -
Joel Reymont