each

Inspired by the Scala library of the same name, each is a Template Haskell library that transforms expressions containing invocations of impure subexpressions into calls to fmap, <*>, join, etc. Just mark your impure subexpressions with bind or ~! and they will be called appropriately, as in this small demo:

ghci> $(each [| "Hello, " ++ (~! getLine) |])
World              <--[keyboard input]
"Hello, World"

In this case, the code is translated into fmap ((++) "Hello, ") getLine

We currently have support for

• Normal function application like f x y
• Infix operator application like x + y, including sections like (+ y)
• Type signatures like x :: t

Support for more constructs is coming.

More demos

A more detailed demo:

ghci> :{
    | $(each [|
    |   "Hey it works"
    |   ++ show (length $
    |     "something"
    |     ++ (~! readFile "/etc/issue")
    |     ++ (~! readFile "/etc/issue.net"))
    | |])
    | :}
"Hey it works64"

Nested binds also work as expected.

ghci> prompt str = putStrLn str *> getLine
ghci> $(each [| "Nah just " ++ (~! prompt ("What's " ++ bind getLine ++ "?")) |])
something          <--[keyboard input]
What's something?
nothing            <--[keyboard input]
"Nah just nothing"