[Haskell-cafe] Re: Haskell Helper
ben.franksen at online.de
Sun Oct 3 18:24:05 EDT 2010
> The problem is there. A function in Hai would be function-name,
> Func stores function name,arguments and body as Strings(I was thinking to
> put Func String String String).
> The parser func that I wrote so far try to parse a function definition,
> not a function call.
> But when I try to store the function on my Map I get a error with somthing
> called 'functional dependencies'(which I don't know what is).
Couldn't match expected type `Hai' against inferred type `[Hai]'
Expected type: Map.Map Hai Hai
Inferred type: Map.Map [Hai] Hai
When using functional dependencies to combine
MonadState (Map.Map [Hai] Hai) m,
arising from a use of `get' at hai1.hs:52:17-19
MonadState (Map.Map Hai Hai) m,
arising from a use of `get' at hai1.hs:47:16-18
When generalising the type(s) for `w'
The type checker tells you that you are using the same Map with different
key types: at 52:17-19 the key has type [Hai], whereas at 47:16-18 it has
The latter is in your Func case:
e <-return $ Map.insert (a :[b]) c d
where you use a :[b] which is the same as [a,b] for the key.
Everywhere else, the key has type Hai. This in itself is questionable: do
you really want to use arbitrary expressions as keys? Usually one would
Map String Hai
representing a map from variable (or function) names to expressions.
For functions you then want
data Hai = ... | Func [String] Hai | ...
Func args body
represents the (anonymous) function with the formal arguments args and the
resulting expression body . The function gets a name by inserting it into
the variable map. This means that a definition
actually defines a variable named "function-name" which, when it gets
looked up in the environment, yields the value Func [arg1,...,argn] body .
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