thereadme/theread.me
1
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity

post(typoclassopedia): Monad Transformers#Composing Monads

Browse Source
This commit is contained in:
Mahdi Dibaiee 2017-10-13 21:36:10 +03:30
parent 78c9fccc72
commit 8b2934f53b
1 changed files with 17 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
_posts/2017-09-27-typoclassopedia-exercise-answers.md
View File

@ -714,3 +714,20 @@ class MonadTrans t where
`t` is of the kind `(* -> *) -> * -> *`, as we see in `(t m) a`, `t` accepts a `Monad` first, which is of type `* -> *`, and then `t` is of the kind `(* -> *) -> * -> *`, as we see in `(t m) a`, `t` accepts a `Monad` first, which is of type `* -> *`, and then
another argument of kind `*`. another argument of kind `*`.
## Composing Monads
### Exercises
1. Implement `join :: M (N (M (N a))) -> M (N a)` given `distrib :: N (M a) -> M (N a)` and assuming `M` and `N` are instances of `Monad`.
```haskell
join :: M (N (M (N a))) -> M (N a)
join m = distrib ((distrib m) >>= join) >>= join
-- one by one
let m :: M (N (M (N a)))
a = distrib m :: N (M (M (N a)))
b = a >>= join :: N (M (N a))
c = distrib b :: M (N (N a))
in c >>= join :: M (N a)
```