Type checking expressions

[Robin Palotai]

I wondered if
https://downloads.haskell.org/~ghc/7.8.1/docs/html/users_guide/defer-type-errors.html
could help.

I tried to click around in the GHC 8.2 tree of
http://stuff.codereview.me/ghc/#ghc/compiler/typecheck/TcErrors.hs?corpus=ghc-8.2.1-rc2&signature&line=120,
but it seems deferring type errors just reports via a different means, and
doesn't change the type-checking behavior. But correct me if I'm wrong.

2018-03-05 17:11 GMT+01:00 Simon Peyton Jones via ghc-devs <
ghc-devs at haskell.org>:

> Always cc ghc-devs!   Bottle-necking on me may well yield a slow
> response!   Or even Haskell-café.
>
>
>
> What is the type of (\x -> [x,y])?   Where y is in scope with type y::a.
> Presumably something like   a -> [a]?  Or is it forall a. a -> [a]?  And
> would your answer change if you had just (\x -> [x,x])?
>
>
>
> Generalisation is tricky, and for terms with non-closed types it is hard
> to know what you need in your use-case.  A type like ‘a’ might be a very
> fine answer!
>
>
>
> A lot depends on precisely what you are trying to do.
>
>
>
> Simon
>
>
>
> *From:* Peter Podlovics [mailto:peter.d.podlovics at gmail.com]
> *Sent:* 05 March 2018 14:54
> *To:* Simon Peyton Jones <simonpj at microsoft.com>
> *Subject:* Re: Type checking expressions
>
>
>
> My main concern with that approach is that it might not give the correct
> type. For example the hsPatType function only gives unconstrained types, so
> it is incorrect for any numeric literal, since it gives "a" instead of "Num
> a => a".
>
> So the question is whether it is possible to retrieve the context of the
> type variables as well. Also this problem may arise in the case of
> expressions as well, that is why I scrapped that approach and tried to type
> check the AST with the TcM monad directly, but without any success.
>
> Could you give me any leads on how to solve this problem?
>
> Thanks in advance,
>
> Peter
>
>
>
> On Mon, Mar 5, 2018 at 9:44 AM, Simon Peyton Jones <simonpj at microsoft.com>
> wrote:
>
> Peter
>
>
>
> My goal is to determine the type of every expression, pattern etc. in the
> syntax tree
>
>
>
> After type checking is complete, the syntax tree is liberally annotated
> with types.
>
>
>
> We do not yet have a function
>
>             hsExprType :: HsExpr Id -> Type
>
> but we do have
>
>             TcHsTyn.hsPatType :: Pat GhcTc -> Type
>
> and you or someone could readily make an equivalent for HsExpr.
>
>
>
> Most type errors are reported by adding an error constraint, but still
> returning an annotated tree.
>
> Some, I’m afraid, are still done in the old way, by throwing an exception
> – so you don’t get back an annotated tree in that case.  But they are
> relatively rare.
>
>
>
> Others must have wanted something like this…
>
>
>
> Simon
>
>
>
> *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Peter
> Podlovics
> *Sent:* 02 March 2018 12:05
> *To:* ghc-devs at haskell.org
> *Subject:* Fwd: Type checking expressions
>
>
>
> Hello everyone,
>
> I would like to ask for some advice regarding the type checker part of GHC.
> My goal is to determine the type of every expression, pattern etc. in the
> syntax tree. Currently the compiler doesn't store this information, so I
> have
> to type check manually. One important aspect is that the program may be
> ill-typed,
> but I still want to extract as much information as possible.
>
> I tried using local type checking functions (eg.: tcInferSigma), but
> whenever
> I used it on an expression that had some "out-of-scope" names in it, it
> failed.
>
> > f xs = length xs
>
> The reason was that xs was not in the local environment.
>
> My question is: how could I provide the necessary local environment for
> these
> type checking functions? Also in the general case, is it possible to
> somehow
> annotate each expression with its type during the type checking?
>
> The motivation for this is that I want to implement a tool that
> automatically
> corrects ill-typed programs based heuristics. For that I need to know the
> types
> of certain AST nodes.
>
> Peter Podlovics
>
>
>
>
>
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