https://github.com/bookshelfdave/monad-transformers-step-by-step
(unofficial) source for Monad Transformers Step by Step by Martin Grabmüller
https://github.com/bookshelfdave/monad-transformers-step-by-step
haskell haskell-exercises haskell-learning monad-transformers monads
Last synced: about 1 month ago
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(unofficial) source for Monad Transformers Step by Step by Martin Grabmüller
- Host: GitHub
- URL: https://github.com/bookshelfdave/monad-transformers-step-by-step
- Owner: bookshelfdave
- License: apache-2.0
- Created: 2020-05-06T15:29:26.000Z (almost 5 years ago)
- Default Branch: master
- Last Pushed: 2024-08-09T06:03:16.000Z (9 months ago)
- Last Synced: 2025-02-27T05:11:40.899Z (about 2 months ago)
- Topics: haskell, haskell-exercises, haskell-learning, monad-transformers, monads
- Language: Haskell
- Homepage:
- Size: 18.6 KB
- Stars: 3
- Watchers: 1
- Forks: 3
- Open Issues: 1
-
Metadata Files:
- Readme: README.md
- Changelog: ChangeLog.md
- License: LICENSE
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README
# Monad Transformers Step by Step
Haskell source for the paper
[Monad Transformers Step by Step](https://page.mi.fu-berlin.de/scravy/realworldhaskell/materialien/monad-transformers-step-by-step.pdf) by Martin Grabmüller, tweaked to work in 2020 and built using [Stack](https://docs.haskellstack.org/en/stable/README/).
## How this repo is organized
- Shared data structureds can be found in [Lib.hs](https://github.com/metadave/monad-transformers-step-by-step/blob/master/src/Lib.hs)
- Each of the `eval0` .. `eval6` implementations can be found in [./src](https://github.com/metadave/monad-transformers-step-by-step/tree/master/src) under a similarly named file.
- Examples of each `eval*` function running can be found in the [test](https://github.com/metadave/monad-transformers-step-by-step/blob/master/test/HUnitTests.hs) file.
## Changes
I've made the following changes to the example source in order for it to compile with the "latest" Haskell tools (as of May 2020).
- In [Lib.hs](https://github.com/metadave/monad-transformers-step-by-step/blob/master/src/Lib.hs), `Eq` was added
to the `deriving` block of `Exp` and `Value`.
- As [`fail` was removed from Monad](https://downloads.haskell.org/%7Eghc/8.8.1/docs/html/users_guide/8.8.1-notes.html#base-library), I've replaced calls to `fail` with `error`.
- I added a `MonadFail` instance for `Identity` in [Lib.hs](https://github.com/metadave/monad-transformers-step-by-step/blob/master/src/Lib.hs)
- more info can be found here https://wiki.haskell.org/MonadFail_Proposal, see the `Adapting old code` section
Without the `MonadFail` instance above, you'll get compilation errors like this:
```
• No instance for (MonadFail Identity)
arising from a do statement
with the failable pattern ‘IntVal i2’
• In a stmt of a 'do' block: IntVal i2 <- eval2c env e2
In the expression:
do IntVal i1 <- eval2c env e1
IntVal i2 <- eval2c env e2
return $ IntVal (i1 + i2)
In an equation for ‘eval2c’:
eval2c env (Plus e1 e2)
= do IntVal i1 <- eval2c env e1
IntVal i2 <- eval2c env e2
return $ IntVal (i1 + i2)
|
52 | IntVal i2 <- eval2c env e2
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
- `Control.Monad.Except` is used in place of `import Control.Monad.Error`, as `Control.Monad.Error` has been [deprecated](https://hackage.haskell.org/package/mtl-2.2.2/docs/Control-Monad-Error.html).
- as the result of this, you'll see `ErrorT` replaced by `ExceptT` in most of the examples.
# Building
```bash
git clone https://github.com/metadave/monad-transformers-step-by-step.git
cd monad-transformers-step-by-step
stack test
# - or-
stack ghci
```
## Interactive Examples
```bash
stack ghci
import qualified Data.Map as Map
eval0 Map.empty exampleExp
-- will display:
IntVal 18
```
# TODO
- Figure out how to test the eval2* functions
# Similar repos
The following repos contain similar implementations, although they don't seem to work in 2020:
- https://github.com/NickAger/monad-transformers-step-by-step
- https://github.com/danbroooks/monad-transformers-step-by-step