[Haskell-cafe] Is there a tutorial interpreter to teach haskell?

[Noah Easterly]

On Tue, Nov 30, 2010 at 12:08 PM, Larry Evans <cppljevans at suddenlink.net>wrote:

>
> so now I must "manually" figure out what the a and b in
> the ap declaration correspond to in the return(:) type:
>
>        m    ( a             ->           b           )
>        __     _                          _
>     1: []     Char          ->     [Char]->[Char]
>     2: []     Char->[Char]  ->          [Char]
>
> A type a -> b -> c is always equivalent to the type a -> (b->c), not (a->b)
-> c.

In particular, breaking down sequence (c:cs) = return (:) `ap` c `ap`
sequence cs

 return (:) :: m (a -> [a] ->[a])
 (\c -> return (:) `ap` c) :: m a -> m ([a] -> [a])
 (\c cs' -> return (:) `ap` c `ap` cs') :: m a -> m [a] -> m [a]

therefore

 sequence :: [ m a ] -> m [a]

Perhaps a special tutorial interpreter would be of use, but I've had some
success simply passing in anonymous functions to ghci's :t operator, since
that lets me simplify a program one bit at a time, inferring the types that
might confuse me.
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