Re: [Haskell-cafe] Re: Type errors, would extensions help?

Here's the desugaring:

do { pattern <- expression ; rest }

desugars to

expression >>= \temp -> case temp of pattern -> do { rest } _ -> fail "Pattern match failure"

(where "temp" is a fresh variable not used elsewhere, and the failure message usually includes source position) Whereas

do { let pattern = expression ; rest }

desugars to

let pattern = expression in do { rest }

-- ryan On Thu, Jan 15, 2009 at 3:26 PM, Lennart Augustsson wrote:

The <- binding is lambda binding (look at how it desugars). Lambda bindings are monomorphic without any type extensions. The monadic 'let' binding is like regular 'let', so it's a point where the type checker does generalization, and so you get (possibly) polymorphic bindings from let.

-- Lennart

On Thu, Jan 15, 2009 at 11:20 PM, Mauricio wrote:

...

Thanks, everything works now.

What should I read to better understand the difference for the type system between using <- and 'let'? That is not intuitive for me.

About layout, I used to filter my code to better fit everyone taste before posting to this list. The filter stoped working due to some problems in 'Language.Haskell', but I'll rewrite it with haskell-src-exts before posting again.

Thanks, Maurício

...

I suggest you start using "let" in your do blocks; both of these problems are solvable with let.

...

Binding with <- instead of "let" makes the type system work harder, and will generally require type annotations & extensions for polymorphic results. (...)

...

Also, is there a reason you hate the layout rule and are using explicit semicolons everywhere?

...

...

...

I have this problem trying to define a function inside a do expression. (...)

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