[Haskell-cafe] Wishful thinking: a text editor that expands function applications into function definitions

Duane Johnson duane.johnson at gmail.com
Thu Apr 2 20:01:44 EDT 2009

So I was thinking about a "killer feature" for a text editor.   
Wouldn't it be neat if you could expand function calls into their  
definitions, in-place?

For example, suppose we have "minus" defined like so, somewhere in  
another file:

> minus (a, b, c) (x, y, z) = (a - x, b - y, c - z)

Later, we make use of the function in our current context:

> let p1 = (1, 2, 3)
>      p2 = (4, 5, 6)
> in p1 `minus` p2

By telling the editor to "expand" the minus, we get a temporary  
replacing of the above with:

> (1 - 4, 2 - 5, 3 - 6)

Another example:

>   parse s = map readLine ls

And supposing that readLine is defined somewhere else, moving the  
cursor to readLine in the line above and "expanding" becomes:

>   parse s = map (\line -> words $ dropWhile (== ' ') line)

This is all pretty standard for the kinds of things we do in Haskell  
to work it out by hand, but is there any reason the parser couldn't do  
this?  I think it would be even harder to do automatically in any  
other language.  Maybe it's already been attempted or done?


Duane Johnson

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