Hey GHC, what-it-do?
what-it-do is a GHC source plugin that rewrites do expressions to trace
all binds. For example, given the following code:
main =
do
a <- return ( 42 :: Int )
b <- return ( a * 2 )
f <- return ( \b -> return ( b - 1 ) )
c <- f ( a * b )
print cWe can trace all binds by using debugDo from what-it-do, and enabling the plugin:
{-# OPTIONS -fplugin=WhatItDo #-}
import WhatItDo (traceDo)
main =
traceDo (do
a <- return 42
b <- return ( a * 2 )
f <- return ( \b -> return ( b - 1 ) )
c <- f ( a * b )
print c)Now, when this code runs, we see:
(Test.hs:5:5-29) a = 42
(Test.hs:6:5-25) b = 84
(Test.hs:8:5-20) c = 3527
3527
Magic!
Wha... how is this possible?
Under the hood, what-it-do has rewritten the original do expression to:
main =
do
a <- return ( 42 :: Int ) >>= \x -> trace ( show x ) ( return x )
b <- return ( a * 2 ) >>= \x -> trace ( show x ) ( return x )
f <- return ( \b -> return ( b - 1 ) )
c <- f ( a * b ) >>= \x -> trace ( show x ) ( return x )
print cNotice that because the plugin has access to the type-checker, we only trace
things that can be shown. The binding to f doesn't get traced, because we
can't show what f is.