monadplus

create table a (isOk BOOLEAN not null);

CREATE or replace FUNCTION get_isOk ( )
returns BOOLEAN as $$
  select isOk from faa;
$$ LANGUAGE SQL;

drop table a;

-- select get_isOk(); -- now fails

monadplus

-- CONSTRAINT kind:
--  * fully-saturated typeclasses
--  * tuples of other CONSTRAINTS
--  * type equalities.

-- In regular haskell this is restricted:
-- 1. We can't mention type variables in the arguments unless they're also in
--   the result.
-- 2. We can't /constrain/ the values of the input using any typeclasses.

monadplus

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
module ClassSynonyms where

-- This is bordersome to write every time..
-- f :: (C1 a, C2 a, C3 a) => ...

-- class synonyms
class (C1 a, C2 a, C3 a) => C a where { }
instance (C1 a, C2 a, C3 a) => C a where { }

monadplus

NixOps Compilation

If the machine running nixOps is linux, it will build it locally, but also obey any build machines configured in /etc/nix/machines.

If nixops is being ran on darwin/windows, it cant build linux things, so it will try to configure the remote machine as a build machine, automatically.

There is a workaround for this if you don't won't to compile on your sever (for example, if the instances is too small it will fail): set a remote build machine over ssh.

To set up a remote build read these links:

monadplus

Profiling in Haskell

Do not get bogged down in microoptimizations before you've assessed any macro optimizations that are available. IO and the choice of algorithm dominate any low level changes you may make. In the end you have to think hard about your code!

Before starting to optimize:

1. Is the -O2 flag on ?
2. Profile: which part of the code is the slow one.
3. Use the best algorithm in that part.
4. Optimize: implement it in the most efficient way.

monadplus

Crossing the Chasm

1. Compeling reason to buy
2. Very specific target
3. Polish whole product
4. Not a leader on the space you are trying to occupy

monadplus

LATERAL JOIN

Data:

                Table "public.orders"
   Column   |            Type             | Modifiers
------------+-----------------------------+-----------
 id         | integer                     | not null
 user_id    | integer                     |

monadplus

Source: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#overlapping-instances

As a more substantial example of the rules in action, consider

instance {-# OVERLAPPABLE #-} context1 => C Int b     where ...  -- (A)
instance {-# OVERLAPPABLE #-} context2 => C a   Bool  where ...  -- (B)
instance {-# OVERLAPPABLE #-} context3 => C a   [b]   where ...  -- (C)
instance {-# OVERLAPPING  #-} context4 => C Int [Int] where ...  -- (D)

monadplus

These are failed experiments to setup a development environment for Python in NixOS.

The working solution can be found: https://github.com/monadplus/python-template-nix

monadplus

{-# LANGUAGE InstanceSigs        #-}
{-# LANGUAGE LambdaCase          #-}
{-# LANGUAGE OverloadedStrings   #-}
{-# LANGUAGE RebindableSyntax    #-}
{-# LANGUAGE ScopedTypeVariables #-}

type Input = String


data ParseResult a =