Tracing to files

The traceFile function appends to the provided file path given as its first argument, the trace message given as its second argument, before returning the third argument as its result.

For example, this returns the value of f x and outputs the message to "/tmp/message".

>>> let x = 123; f = show
>>> traceFile "/tmp/message" ("calling f with x = " ++ show x) (f x)
"123"
>>> readFile "/tmp/message"
"calling f with x = 123\n"

The traceFile function should only be used for debugging, or for monitoring execution. The function is not referentially transparent: its type indicates that it is a pure function but it has the side effect of outputting the trace message.

Like traceFile but uses writeFile instead of appendFile which means will overwrite the contents of the file.

Like traceFile but returns the message instead of a third value.

>>> traceFileId "/tmp/message" "hello"
"hello"
>>> readFile "/tmp/message"
"hello\n"

Like traceFileId but uses writeFile instead of appendFile which means will overwrite the contents of the file.

Like traceFile, but uses show on the argument to convert it to a String.

This makes it convenient for printing the values of interesting variables or expressions inside a function. For example here we print the value of the variables x and y:

>>> let f x y = traceFileShow "/tmp/message" (x,y) (x + y) in f (1+2) 5
8
>>> readFile "/tmp/message"
"(3,5)\n"

Like traceFileShow but uses writeFile instead of appendFile which means will overwrite the contents of the file.

Like traceFileShow but returns the shown value instead of a third value.

>>> traceFileShowId "/tmp/message" (1+2+3, "hello" ++ "world")
(6,"helloworld")
>>> readFile "/tmp/message"
"(6,\"helloworld\")\n"

Like traceFileShowId but uses writeFile instead of appendFile which means will overwrite the contents of the file.

Like traceFile, but outputs the result of calling a function on the argument.

>>> traceFileWith "/tmp/message" fst ("hello","world")
("hello","world")
>>> readFile "/tmp/message"
"hello\n"

Like traceFileWith but uses writeFile instead of appendFile which means will overwrite the contents of the file.

Like traceFileWith, but uses show on the result of the function to convert it to a String.

>>> traceFileShowWith "/tmp/message" length [1,2,3]
[1,2,3]
>>> readFile "/tmp/message"
"3\n"

Like traceFileWith but uses writeFile instead of appendFile which means will overwrite the contents of the file.

Like traceFile but returning unit in an arbitrary Applicative context. Allows for convenient use in do-notation.

>>> :{
do
    x <- Just 3
    traceFileM "/tmp/message" ("x: " ++ show x)
    y <- pure 12
    traceFileM "/tmp/message" ("y: " ++ show y)
    pure (x*2 + y)
:}
Just 18
>>> readFile "/tmp/message"
"x: 3\ny: 12\n"

Like traceFileM but uses writeFile instead of appendFile which means will overwrite the contents of the file.

Like traceFileM, but uses show on the argument to convert it to a String.

>>> :{
do
    x <- Just 3
    traceFileShowM "/tmp/message" x
    y <- pure 12
    traceFileShowM "/tmp/message" y
    pure (x*2 + y)
:}
Just 18
>>> readFile "/tmp/message"
"3\n12\n"

Like traceFileShowM but uses writeFile instead of appendFile which means will overwrite the contents of the file.