Re: [Haskell-beginners] Compiler can't deduce Bool as instance of ToField

15 Aug 2013


      On 15 Aug 2013, at 19:49, Bryan Vicknair  wrote:
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Bryan Vicknair:
postgresql-simple declares Bool as an instance of the ToField class.  The
compiler can't deduce that given this simple code however:
import Database.PostgreSQL.Simple.ToField (ToField(..))
foo :: (ToField a) => a
 foo = True
It fails with this error:
Db.hs:64:7:
     Could not deduce (a ~ Bool)
     from the context (ToField a)
       bound by the type signature for foo :: ToField a => a
       at Db.hs:63:8-23
       `a' is a rigid type variable bound by
           the type signature for foo :: ToField a => a at Db.hs:63:8
     In the expression: True
     In an equation for `foo': foo = True
 Failed, modules loaded: none.
What am I missing?
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Oliver Charles 
You have declared that foo is *any* type that has a ToField instance,
allowing the caller of foo to determine the type at their will. However,
your implementation of foo is more specific and requires a is actually Bool
and nothing else.
On 14 Aug 2013 10:24, "Bryan Vicknair"  wrote:
I see.  I was confused, because the following works:
bar :: (Num a) => a
 bar = 5
I guess it is because the literal '5' could be an Int or a Float, and the type
system doesn't know.
When I saw that the following does not work...::
bar :: (Show a) => a
 bar = False
...it made a bit more sense.  It seems that if a literal can be considered part
of a typeclass in more than one way, you can declare that literal to be of that
typeclass.  However, if there is only one way for a literal to belong to a
typeclass, then you can't.
That’s not it here.  What you’re saying in the first case is “no matter what numeric type you want, I can provide it”.  This is true, because the compiler can decide that 5 can indeed be any numeric type.  In the latter case you make the promise “no matter what type you want, as long as you can show it, I can provide it”, but it’s false.  I can say “well okay, give me an Int” and your function can not provide it to me, despite telling me in its type signature that it can.

Tom Davie