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Today's Topics:
1. help writing a simple api which queries for a resource and
displays this info (xu xiut)
2. type T' = T a b c (Imants Cekusins)
3. Re: type T' = T a b c (Imants Cekusins)
4. Re: type T' = T a b c (Imants Cekusins)
----------------------------------------------------------------------
Message: 1
Date: Sun, 21 Aug 2016 16:05:52 -0600
From: xu xiut <xiut...@gmail.com>
To: beginners@haskell.org
Subject: [Haskell-beginners] help writing a simple api which queries
for a resource and displays this info
Message-ID:
<CANVqfJEOwijry_o_n7KSF9sc1sni-RmsFU33EX=+rcoirq_...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hello, I am wanting to store something in a database, then retrieve it.
That is my entire ambition for the day.
Technologies chosen:
spock:
https://github.com/agrafix/Spock
rethinkdb driver:
https://github.com/AtnNn/haskell-rethinkdb
I chose to go with Rethinkdb, but i'm open to using Postgresql.
If someone requests for /companies, I want to list all the companies in the
"companies" table.
```
-- ghci
λ> companies <- run h $ table "companies" :: IO [Datum]
λ> companies
[{"name":"tesla","id":"7781ee7e-1e43-4608-bb96-fe10cac3b53a"}]
λ> firstCompany <- run h $ table "companies" ! "name" :: IO [Datum]
λ> firstCompany
["tesla"]
λ> :info Datum
data Datum
= Null
| Bool Bool
| String Text
| Number Double
| Array Database.RethinkDB.Datum.Array
| Object Database.RethinkDB.Datum.Object
| Time time-1.5.0.1:Data.Time.LocalTime.LocalTime.ZonedTime
| Point LonLat
| Line GeoLine
| Polygon GeoPolygon
| Binary bytestring-0.10.6.0:Data.ByteString.Internal.ByteString
-- Defined in ‘Database.RethinkDB.Datum’
instance Eq Datum -- Defined in ‘Database.RethinkDB.Datum’
instance Ord Datum -- Defined in ‘Database.RethinkDB.Datum’
instance Show Datum -- Defined in ‘Database.RethinkDB.Datum’
instance Expr Datum -- Defined in ‘Database.RethinkDB.ReQL’
instance Result Datum -- Defined in ‘Database.RethinkDB.Driver’
instance ToDatum Datum -- Defined in ‘Database.RethinkDB.Datum’
instance FromDatum Datum -- Defined in ‘Database.RethinkDB.Datum’
15:40 < lpaste> xuxu revised “investigating the Datum
data type”: “investigating the Datum data
type” at http://lpaste.net/179391
```
I don't know how to display the result.
>From Spock's readme, here's an example where I don't need to query the
database:
```
main =
runSpock 3000 $ spockT id $
do get "companies" $
text $ T.concat ["tesla", "google"]
```
How to convert that into something a little more practial where I do
utilize a db?
Here's what I have right now, but I'm sure it doesn't compile
```
{-# LANGUAGE OverloadedStrings #-}
import Web.Spock
import qualified Data.Text as T
import qualified Database.RethinkDB as R
import qualified Database.RethinkDB.NoClash
default (Datum, ReQL, String, Int, Double)
main :: IO ()
main =
-- 1. i don't know if the following line will work
let h = connect "localhost" 28015 Nothing in
runSpock 3000 $ spockT id $
-- 2. list all companies
do get "companies" $
text $ T.concat -- something
```
I don't exactly know how "text" works.
If I'm reading
https://github.com/agrafix/Spock/blob/1ee54503ad67f62af795a31772040f56f7ae08fd/Spock-core/src/Web/Spock/Core.hs#L55
correctly, spockT is using a reader monad. I don't know how it works with
the Text type, but I have a suspicion I need to convert Rethink's Datum
type into a Text type.
I would be absolutely thrilled if anyone is able to help with this.
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Message: 2
Date: Mon, 22 Aug 2016 12:01:08 +0200
From: Imants Cekusins <ima...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] type T' = T a b c
Message-ID:
<CAP1qinZ=b6oOfnrp=relgov61t4wruzhwbg49m5dfy9nne4...@mail.gmail.com>
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*problem*:
given type T with a few parameters e.g.
T a b c
some functions where this type is passed to, are agnostic to a, b, c
i.e. accept T with any of a, b, c
processT::T a b c -> ()
when either number of these parameters or T change (c is dropped or d is
added: T a b c d), signatures of every function where T is passed to, need
to be updated.
*question*:
is this possible to specify type synonym T' so that it may be passed to
parameter (a,b,c) - agnostic functions so:
processT::T' -> ()
?
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Message: 3
Date: Mon, 22 Aug 2016 12:29:14 +0200
From: Imants Cekusins <ima...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] type T' = T a b c
Message-ID:
<CAP1qinbgcY1X=4+ga95_f1o4cbqfl47wbyqrjyxbcrb5z31...@mail.gmail.com>
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just came across this:
https://downloads.haskell.org/~ghc/7.0.3/docs/html/users_guide/data-type-extensions.html
this seems to work:
ExistentialQuantification
Rank2Types
type T' = forall a b. (T a b)
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Message: 4
Date: Mon, 22 Aug 2016 13:32:15 +0200
From: Imants Cekusins <ima...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] type T' = T a b c
Message-ID:
<CAP1qina_z53Q3h2RJRJsuOMUutRJWdH3pCO5chYA=iejunf...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
T' and T a b seem to not mix well.
T' can not be passed to a function expecting T a b and vice versa
any suggestions?
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