haskell do構文

失敗

import System.Environment
main::IO()
main=
  do
    args<-getArgs
    argsl<-length args  --Couldn't match expected type ‘IO a0’ with actual type ‘Int’
    (print.show) argsl

成功

import System.Environment
main::IO()
main=
  do
    args<-getArgs
    let argsl=length args
    (print.show) argsl

import System.Environment
main::IO()
main=
  do
    argsl<-length<$>getArgs
    (print.show) argsl

参考ページ

qiita.com