Using purescript-debug:
import Debug.Trace (trace, traceShow, traceAny, spy, traceA, traceShowA, traceAnyA, traceShowM, traceAnyM)
First we have the “plain” trace functions: trace, traceShow and traceAny.
Each of them logs a message to the console and then returns a value. The return value is thunked so it is not evaluated until after the message has been printed, to preserve a predictable console output.
The difference is that trace takes a String, traceShow a Showable value, and traceAny takes any value, logging its underlying representation.
trace :: forall a. String -> (Unit -> a) -> a
traceShow :: forall a b. Show a => a -> (Unit -> b) -> b
traceAny :: forall a b. a -> (Unit -> b) -> b
Some examples:
foo :: Int -> Int
foo x = do
trace ("Called foo with " <> show x) \_ ->
x + 2
bar :: Int -> Int
bar x = do
traceShow x \_ ->
x * 2
newtype Baz = Baz Int
baz :: Baz -> String
baz x = do
traceAny x \_ ->
42
If you’re in an Applicative context, you can use the traceA variants that return the unit value of the Applicative a:
traceA :: forall a. Applicative a => String -> a Unit
traceShowA :: forall a b. Show b => Applicative a => b -> a Unit
traceAnyA :: forall a b. Applicative a => b -> a Unit
A simple example:
main :: Eff () Unit
main = do
-- some computations
traceAnyA "Test"
-- other computations
Then we have traceAnyM and traceShowM which are used inside of monadic chains:
traceShowM :: forall m a. Show a => Monad m => a -> m a
traceAnyM :: forall m a. Show a => Monad m => a -> m a
A simple example:
foo :: Maybe (Array Int) -> Int
foo mbArray = fromMaybe zero
$ mbArray
>>= traceAnyM
>>= head
>>= traceAnyM
-- Under each invocation I'm writing the console output when calling `foo` in psci:
foo $ Just [1, 2]
-- [1, 2] (first traceAnyM)
-- 1 (second traceAnyM)
-- 1 (return value)
foo $ Just []
-- [] (first traceAnyM)
-- 0 (return value from fromMaybe)
foo Nothing
-- 0 (return value, there's no logging)
Finally there’s spy, which logs a value and returns it:
spy :: forall a. a -> a
This example will log 3 and then assign it to foo:
main = do
--- stuff
let foo = spy 3
-- other stuff