# Monad comprehensions

After a long absence, monad comprehensions are back, thanks to George Giorgidze and his colleagues. With `{-# LANGUAGE MonadComprehensions #-}` the comprehension `[ f x | x <- xs, x>4 ]` is interpreted in an arbitrary monad, rather than being restricted to lists. Not only that, it also generalises nicely for parallel/zip and SQL-like comprehensions. The aforementioned generalisations can be turned on by enabling the `MonadComprehensions` extension in conjunction with the `ParallelListComp` and `TransformListComp` extensions.

Rebindable syntax is fully supported for standard monad comprehensions with generators and filters. We also plan to allow rebinding of the parallel/zip and SQL-like monad comprehension notations.

For further details and usage examples, see the paper "Bringing back monad comprehensions" http://db.inf.uni-tuebingen.de/staticfiles/publications/haskell2011.pdf at the 2011 Haskell Symposium.

See ticket #4370.

## Translation rules

Variables : x and y Expressions : e, f and g Patterns : w Qualifiers : p, q and r

The main translation rule for monad comprehensions.

[ e | q ] = [| q |] >>= (return . (\q_v -> e))

`(.)_v` rules. Note that `_v` is a postfix rule application.

(w <- e)_v = w (let w = d)_v = w (g)_v = () (p , q)_v = (p_v,q_v) (p | v)_v = (p_v,q_v) (q, then f)_v = q_v (q, then f by e)_v = q_v (q, then group by e using f)_v = q_v (q, then group using f)_v = q_v

`[|.|]` rules.

[| w <- e |] = e [| let w = d |] = return d [| g |] = guard g [| p, q |] = ([| p |] >>= (return . (\p_v -> [| q |] >>= (return . (\q_v -> (p_v,q_v)))))) >>= id [| p | q |] = mzip [| p |] [| q |] [| q, then f |] = f [| q |] [| q, then f by e |] = f (\q_v -> e) [| q |] [| q, then group by e using f |] = (f (\q_v -> e) [| q |]) >>= (return . (unzip q_v)) [| q, then group using f |] = (f [| q |]) >>= (return . (unzip q_v))

`unzip (.)` rules. Note that `unzip` is a desugaring rule (i.e., not a function to be included in the generated code).

unzip () = id unzip x = id unzip (w1,w2) = \e -> ((unzip w1) (e >>= (return .(\(x,y) -> x))), (unzip w2) (e >>= (return . (\(x,y) -> y))))

### Examples

Some translation examples using the do notation to avoid things like pattern matching failures are:

[ x+y | x <- Just 1, y <- Just 2 ] -- translates to: do x <- Just 1 y <- Just 2 return (x+y)

Transform statements:

[ x | x <- [1..], then take 10 ] -- translates to: take 10 (do x <- [1..] return x)

Grouping statements (note the change of types):

[ (x :: [Int]) | x <- [1,2,1,2], then group by x ] :: [[Int]] -- translates to: do x <- mgroupWith (\x -> x) [1,2,1,2] return x

Parallel statements:

[ x+y | x <- [1,2,3] | y <- [4,5,6] ] -- translates to: do (x,y) <- mzip [1,2,3] [4,5,6] return (x+y)

Note that the actual implementation is **not** using the `do`-Notation, it's only used here to give a basic overview about how the translation works.

## Implementation details

Monad comprehensions had to change the `StmtLR` data type in the `hsSyn/HsExpr.lhs` file in order to be able to lookup and store all functions required to desugare monad comprehensions correctly (e.g. `return`, `(>>=)`, `guard` etc). Renaming is done in `rename/RnExpr.lhs` and typechecking in `typecheck/TcMatches.lhs`. The main desugaring is done in `deSugar/DsListComp.lhs`. If you want to start hacking on monad comprehensions I'd look at those files first.

Some things you might want to be aware of:

[todo]