[Haskell-beginners] A one- or two-page diagram of how Haskell works?

[Steven Leiva]

Wonderful question. I would love to see something like this too.

What kind of confusion are you running into with Haskell? Are they of the
type error variety, or do you have a well-typed program that doesn't do
what you want?

*If* it is the type error stuff, I would recommend that you give the
compiler information about what you think the types are. The reason is that
type information can flow from very far away places than where you are
getting an error. For example, I was writing this code the other day:

*for eIdpData \ $(schoolId, districtId, tenantId) -> pure $ Right 1*

The type of for is *for :: (Traversable t, Applicative f) => t a -> (a -> f
b) -> f (t b)*

In my case, I knew the following:

*t ~ Either SyncFailure*
*a ~ IdpData*
*f ~ WriterT Logs m*
*b ~ Int*

Once I do replace type variables with concrete types / type constructors,
it becomes clear that my function argument should return an *Int* in some
structure, and instead I am returning an *Either a Int*. This helps me
figure out the problem.

Writing out takes a lot longer than actually doing it. Anyway, the key is
to provide GHC with the information you think you know; this will prevent
type inference from allowing type information to flow from far away places,
and the error will become clearer or at the very least more localized.

On Wed, Jun 23, 2021 at 11:42 PM Michael Turner <
michael.eugene.turner at gmail.com> wrote:

> When I write C, or even C++, I have a mental model of how execution
> will proceed.
>
> When I write Prolog, but get confused, I run a kind of skeletal
> inference algorithm in my head and the confusion usually clears up. I
> can imagine how things are stored and what's done with them. I can see
> /through/ the code to the machine.
>
> With Haskell, I still feel blind.
>
> Has anyone summarized it all in a chart where I can look at it and
> think, "Ah, OK, GHC is taking this line and thinking of it THIS way"?
> If someone wanted to write an interpreter for Haskell, would there be
> a way for them to see how it would basically need to work, in one
> chart?
>
> Regards,
> Michael Turner
> Executive Director
> Project Persephone
> 1-25-33 Takadanobaba
> Shinjuku-ku Tokyo 169-0075
> Mobile: +81 (90) 5203-8682
> turner at projectpersephone.org
>
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-- 
Steven Leiva
305.528.6038
leiva.steven at gmail.com
http://www.linkedin.com/in/stevenleiva
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