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:

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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