Haskell: Turtle: Managing Shell type

Question

This a working snippet:

import Turtle
...
groom :: FilePath -> IO ()
groom src = do
  view (ls src)
...

I can see a list of paths on the console. Actually I'd like to have something like [FilePath] for use, e.g.:

treeCount :: FilePath -> Int
treeCount src = length (lstree src)

Naturally, it won't compile, lstree being what it is:

lstree :: FilePath -> Shell FilePath

What is the correct way to treat this Shell thing? It's a newbie question, sorry.

@melpomene https://hackage.haskell.org/package/turtle it seems — Del, Dec 10 '17 at 18:32
`length :: t a -> Int`; used mainly on lists. Pretty polymorphic, but not enough for `Shell FilePath` :) — Alexey Orlov, Dec 10 '17 at 18:36
http://hackage.haskell.org/package/turtle-1.4.5/docs/Turtle-Prelude.html#v:sort looks close. — Gurkenglas, Dec 10 '17 at 18:40
Does `length` have typeclass constraints? Which package is it from? — Del, Dec 10 '17 at 18:41
`length` is from `Data.Foldable`, it's one of the bedrock tools. It's beside the point, though. One has understand `Shell` to solve this problem. I've no idea so far. — Alexey Orlov, Dec 10 '17 at 18:46
http://hackage.haskell.org/package/turtle-1.4.5/docs/Turtle-Shell.html# says it has `IO a -> Shell a`, so it can't possibly be `Foldable`. — Gurkenglas, Dec 10 '17 at 19:32

score 1 · Accepted Answer · answered Dec 10 '17 at 18:47

1

I haven't actually tried this, but just looking at the type signatures the following might work:

import qualified Control.Foldl as F

treeCount :: FilePath -> IO Int
treeCount src = fold (lstree src) F.length

Fold with F.list to get [FilePath] instead.

answered Dec 10 '17 at 18:47

melpomene

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Couldn't match expected type ‘Shell FilePath -> Fold a1 Int -> Int’ with actual type ‘Fold a0 [a0]’ • The function ‘F.list’ is applied to two arguments, but its type ‘Fold a0 [a0]’ has none In the expression: F.list (lstree src) F.length In an equation for ‘treeCount’: `treeCount src = F.list (lstree src) F.length` – Alexey Orlov Dec 10 '17 at 19:01
@AlexeyOrlov That needs to be `fold (lstree src) F.list`. – melpomene Dec 10 '17 at 19:06
No instance for (MonadIO []) arising from a use of ‘fold’ – Alexey Orlov Dec 10 '17 at 19:12
@AlexeyOrlov Did you give it the wrong type signature? It should return `IO [FilePath]`. – melpomene Dec 10 '17 at 19:14
It compiles, thanks! I'm taking a timeout: I have to grok it. – Alexey Orlov Dec 10 '17 at 19:18
@AlexeyOrlov I'm not familiar with Shell, but there seems to be a lot going on here. `Shell` is a monad, where `Shell a` stands for a stream of `a` values, each produced with a side effect. The library provides functions like `Turtle.Shell.fold` to "convert" a `Shell` computation into an `IO` one, as long as a folding function is provided to "compress" the stream into a single value (e.g. `F.length` above). – chi Dec 10 '17 at 19:25
Thanks again! I'm falling behind, though. I hope for tomorrow. – Alexey Orlov Dec 10 '17 at 19:31
I am not the first one to be trapped by IO monad: https://stackoverflow.com/questions/11467066/how-to-get-normal-value-from-io-action-in-haskell – Alexey Orlov Dec 11 '17 at 09:21
There is even an icon: https://photos.smugmug.com/Humor/Lambdacats/trapd-in-IO-monad-plz-help/960526421_MnNqB-S-1.jpg – Alexey Orlov Dec 11 '17 at 09:22

Haskell: Turtle: Managing Shell type

1 Answers1